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1. Introduction

This document formally specifies the core features of the CSS Object Model (CSSOM). Other documents in the CSSOM family of specifications as well as other CSS related specifications define extensions to these core features.

The core features of the CSSOM are oriented towards providing basic capabilities to author-defined scripts to permit access to and manipulation of style related state information and processes.

The features defined below are fundamentally based on prior specifications of the W3C DOM Working Group, primarily [DOM]. The purposes of the present document are (1) to improve on that prior work by providing more technical specificity (so as to improve testability and interoperability), (2) to deprecate or remove certain less-widely implemented features no longer considered to be essential in this context, and (3) to newly specify certain extensions that have been or expected to be widely implemented.

2. Terminology

This specification employs certain terminology from the following documents: DOM, HTML, CSS Syntax, Encoding, URL, Fetch, Associating Style Sheets with XML documents and XML. [DOM] [HTML] [CSS3SYN] [ENCODING] [URL] [FETCH] [XML-STYLESHEET] [XML]

When this specification talks about object A where A is actually an interface, it generally means an object implementing interface A.

The terms set and unset to refer to the true and false values of binary flags or variables, respectively. These terms are also used as verbs in which case they refer to mutating some value to make it true or false, respectively.

The term supported styling language refers to CSS.

Note: If another styling language becomes supported in user agents, this specification is expected to be updated as necessary.

The term supported CSS property refers to a CSS property that the user agent implements, including any vendor-prefixed properties, but excluding custom properties. A supported CSS property must be in its lowercase form for the purpose of comparisons in this specification.

In this specification the ::before and ::after pseudo-elements are assumed to exist for all elements even if no box is generated for them.

When a method or an attribute is said to call another method or attribute, the user agent must invoke its internal API for that attribute or method so that e.g. the author can’t change the behavior by overriding attributes or methods with custom properties or functions in ECMAScript.

Unless otherwise stated, string comparisons are done in a case-sensitive manner.

2.1. Common Serializing Idioms

To escape a character means to create a string of "\" (U+005C), followed by the character.

To escape a character as code point means to create a string of "\" (U+005C), followed by the Unicode code point as the smallest possible number of hexadecimal digits in the range 0-9 a-f (U+0030 to U+0039 and U+0061 to U+0066) to represent the code point in base 16, followed by a single SPACE (U+0020).

To serialize an identifier means to create a string represented by the concatenation of, for each character of the identifier:

• If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD).
• If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, then the character escaped as code point.
• If the character is the first character and is in the range [0-9] (U+0030 to U+0039), then the character escaped as code point.
• If the character is the second character and is in the range [0-9] (U+0030 to U+0039) and the first character is a "-" (U+002D), then the character escaped as code point.
• If the character is the first character and is a "-" (U+002D), and there is no second character, then the escaped character.
• If the character is not handled by one of the above rules and is a non-ASCII ident code point, is "-" (U+002D) or "_" (U+005F), or is in one of the ranges [0-9] (U+0030 to U+0039), [A-Z] (U+0041 to U+005A), or [a-z] (U+0061 to U+007A), then the character itself.
• Otherwise, the escaped character.

To serialize a string means to create a string represented by '"' (U+0022), followed by the result of applying the rules below to each character of the given string, followed by '"' (U+0022):

• If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD).
• If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, the character escaped as code point.
• If the character is '"' (U+0022) or "\" (U+005C), the escaped character.
• Otherwise, the character itself.

Note: "'" (U+0027) is not escaped because strings are always serialized with '"' (U+0022).

To serialize a URL means to create a string represented by "url(", followed by the serialization of the URL as a string, followed by ")".

To serialize a LOCAL means to create a string represented by "local(", followed by the serialization of the LOCAL as a string, followed by ")".

To serialize a comma-separated list concatenate all items of the list in list order while separating them by ", ", i.e., COMMA (U+002C) followed by a single SPACE (U+0020).

To serialize a whitespace-separated list concatenate all items of the list in list order while separating them by " ", i.e., a single SPACE (U+0020).

Note: When serializing a list according to the above rules, extraneous whitespace is not inserted prior to the first item or subsequent to the last item. Unless otherwise specified, an empty list is serialized as the empty string.

3. CSSOMString

Most strings in CSSOM interfaces use the CSSOMString type. Each implementation chooses to define it as either USVString or DOMString:

typedef USVString CSSOMString;

Or, alternatively:

typedef DOMString CSSOMString;

4. Media Queries

Media queries are defined by [MEDIAQUERIES]. This section defines various concepts around media queries, including their API and serialization form.

4.1. Parsing Media Queries

To parse a media query list for a given string s into a media query list is defined in the Media Queries specification. Return the list of media queries that the algorithm defined there gives.

Note: A media query that ends up being "ignored" will turn into "not all".

To parse a media query for a given string s means to follow the parse a media query list steps and return null if more than one media query is returned or a media query if a single media query is returned.

Note: Again, a media query that ends up being "ignored" will turn into "not all".

4.2. Serializing Media Queries

To serialize a media query list run these steps:

1. If the media query list is empty, then return the empty string.
2. Serialize each media query in the list of media queries, in the same order as they appear in the media query list, and then serialize the list.

To serialize a media query let s be the empty string, run the steps below:

1. If the media query is negated append "not", followed by a single SPACE (U+0020), to s.
2. Let type be the serialization as an identifier of the media type of the media query, converted to ASCII lowercase.
3. If the media query does not contain media features append type, to s, then return s.
4. If type is not "all" or if the media query is negated append type, followed by a single SPACE (U+0020), followed by "and", followed by a single SPACE (U+0020), to s.
5. Then, for each media feature:
   1. Append a "(" (U+0028), followed by the media feature name, converted to ASCII lowercase, to s.
   2. If a value is given append a ":" (U+003A), followed by a single SPACE (U+0020), followed by the serialized media feature value, to s.
   3. Append a ")" (U+0029) to s.
   4. If this is not the last media feature append a single SPACE (U+0020), followed by "and", followed by a single SPACE (U+0020), to s.
6. Return s.

not screen and (min-WIDTH:5px) AND (max-width:40px)

not screen and (min-width: 5px) and (max-width: 40px)

all and (color) and (color)

(color) and (color)

4.2.1. Serializing Media Feature Values

This should probably be done in terms of mapping it to serializing CSS values as media features are defined in terms of CSS values after all.

To serialize a media feature value named v locate v in the first column of the table below and use the serialization format described in the second column:

Other specifications can extend this table and vendor-prefixed media features can have custom serialization formats as well.

4.3. Comparing Media Queries

To compare media queries m1 and m2 means to serialize them both and return true if they are a case-sensitive match and false if they are not.

4.4. The MediaList Interface

An object that implements the MediaList interface has an associated collection of media queries.

[Exposed=Window]
interface MediaList {
  stringifier attribute [LegacyNullToEmptyString] CSSOMString mediaText;
  readonly attribute unsigned long length;
  getter CSSOMString? item(unsigned long index);
  undefined appendMedium(CSSOMString medium);
  undefined deleteMedium(CSSOMString medium);
};

The object’s supported property indices are the numbers in the range zero to one less than the number of media queries in the collection of media queries represented by the collection. If there are no such media queries, then there are no supported property indices.

To create a MediaList object with a string text, run the following steps:

1. Create a new MediaList object.
2. Set its mediaText attribute to text.
3. Return the newly created MediaList object.

The mediaText attribute, on getting, must return a serialization of the collection of media queries. Setting the mediaText attribute must run these steps:

1. Empty the collection of media queries.
2. If the given value is the empty string, then return.
3. Append all the media queries as a result of parsing the given value to the collection of media queries.

The item(index) method must return a serialization of the media query in the collection of media queries given by index, or null, if index is greater than or equal to the number of media queries in the collection of media queries.

The length attribute must return the number of media queries in the collection of media queries.

The appendMedium(medium) method must run these steps:

1. Let m be the result of parsing the given value.
2. If m is null, then return.
3. If comparing m with any of the media queries in the collection of media queries returns true, then return.
4. Append m to the collection of media queries.

The deleteMedium(medium) method must run these steps:

1. Let m be the result of parsing the given value.
2. If m is null, then return.
3. Remove any media query from the collection of media queries for which comparing the media query with m returns true. If nothing was removed, then throw a NotFoundError exception.

5. Selectors

Selectors are defined in the Selectors specification. This section mainly defines how to serialize them.

5.1. Parsing Selectors

To parse a group of selectors means to parse the value using the selectors_group production defined in the Selectors specification and return either a group of selectors if parsing did not fail or null if parsing did fail.

5.2. Serializing Selectors

To serialize a group of selectors serialize each selector in the group of selectors and then serialize a comma-separated list of these serializations.

To serialize a selector let s be the empty string, run the steps below for each part of the chain of the selector, and finally return s:

1. If there is only one simple selector in the compound selectors which is a universal selector, append the result of serializing the universal selector to s.
2. Otherwise, for each simple selector in the compound selectors that is not a universal selector of which the namespace prefix maps to a namespace that is not the default namespace serialize the simple selector and append the result to s.
3. If this is not the last part of the chain of the selector append a single SPACE (U+0020), followed by the combinator ">", "+", "~", ">>", "||", as appropriate, followed by another single SPACE (U+0020) if the combinator was not whitespace, to s.
4. If this is the last part of the chain of the selector and there is a pseudo-element, append "::" followed by the name of the pseudo-element, to s.

To serialize a simple selector let s be the empty string, run the steps below, and finally return s:

type selector

universal selector

1. If the namespace prefix maps to a namespace that is not the default namespace and is not the null namespace (not in a namespace) append the serialization of the namespace prefix as an identifier, followed by a "|" (U+007C) to s.
2. If the namespace prefix maps to a namespace that is the null namespace (not in a namespace) append "|" (U+007C) to s.
3. If this is a type selector append the serialization of the element name as an identifier to s.
4. If this is a universal selector append "*" (U+002A) to s.

attribute selector

1. Append "[" (U+005B) to s.
2. If the namespace prefix maps to a namespace that is not the null namespace (not in a namespace) append the serialization of the namespace prefix as an identifier, followed by a "|" (U+007C) to s.
3. Append the serialization of the attribute name as an identifier to s.
4. If there is an attribute value specified, append "=", "~=", "|=", "^=", "$=", or "*=" as appropriate (depending on the type of attribute selector), followed by the serialization of the attribute value as a string, to s.
5. If the attribute selector has the case-sensitivity flag present, append " i" (U+0020 U+0069) to s.
6. Append "]" (U+005D) to s.

class selector

Append a "." (U+002E), followed by the serialization of the class name as an identifier to s.

ID selector

Append a "#" (U+0023), followed by the serialization of the ID as an identifier to s.

pseudo-class

If the pseudo-class does not accept arguments append ":" (U+003A), followed by the name of the pseudo-class, to s.

Otherwise, append ":" (U+003A), followed by the name of the pseudo-class, followed by "(" (U+0028), followed by the value of the pseudo-class argument(s) determined as per below, followed by ")" (U+0029), to s.

:lang()

The serialization of a comma-separated list of each argument’s serialization as a string, preserving relative order.

:nth-child()

:nth-last-child()

:nth-of-type()

:nth-last-of-type()

The result of serializing the value using the rules to serialize an <a-n-plus-b> value.

:not()

The result of serializing the value using the rules for serializing a group of selectors.

6. CSS

6.1. CSS Style Sheets

A CSS style sheet is an abstract concept that represents a style sheet as defined by the CSS specification. In the CSSOM a CSS style sheet is represented as a CSSStyleSheet object.

CSSStyleSheet(options)

When called, execute the steps to create a constructed CSSStyleSheet given options and return the result.

To create a constructed CSSStyleSheet

given CSSStyleSheetInit options, run these steps:

1. Construct a new CSSStyleSheet object sheet.
2. Set sheet’s location to the base URL of the associated Document for the current global object.
3. Set sheet’s stylesheet base URL to the baseURL attribute value from options.
4. Set sheet’s parent CSS style sheet to null.
5. Set sheet’s owner node to null.
6. Set sheet’s owner CSS rule to null.
7. Set sheet’s title to the empty string.
8. Unset sheet’s alternate flag.
9. Set sheet’s origin-clean flag.
10. Set sheet’s constructed flag.
11. Set sheet’s Constructor document to the associated Document for the current global object.
12. If the media attribute of options is a string, create a MediaList object from the string and assign it as sheet’s media. Otherwise, serialize a media query list from the attribute and then create a MediaList object from the resulting string and set it as sheet’s media.
13. If the disabled attribute of options is true, set sheet’s disabled flag.
14. Return sheet.

A CSS style sheet has a number of associated state items:

type

The literal string "text/css".

location

Specified when created. The absolute-URL string of the first request of the CSS style sheet or null if the CSS style sheet was embedded. Does not change during the lifetime of the CSS style sheet.

parent CSS style sheet

Specified when created. The CSS style sheet that is the parent of the CSS style sheet or null if there is no associated parent.

owner node

Specified when created. The DOM node associated with the CSS style sheet or null if there is no associated DOM node.

owner CSS rule

Specified when created. The CSS rule in the parent CSS style sheet that caused the inclusion of the CSS style sheet or null if there is no associated rule.

media

Specified when created. The MediaList object associated with the CSS style sheet.

If this property is specified to a string, the media must be set to the return value of invoking create a MediaList object steps for that string.

If this property is specified to an attribute of the owner node, the media must be set to the return value of invoking create a MediaList object steps for the value of that attribute. Whenever the attribute is set, changed or removed, the media’s mediaText attribute must be set to the new value of the attribute, or to null if the attribute is absent.

Note: Changing the media’s mediaText attribute does not change the corresponding attribute on the owner node.

Note: The owner node of a CSS style sheet, if non-null, is the node whose associated CSS style sheet is the CSS style sheet in question, when the CSS style sheet is added.

title

Specified when created. The title of the CSS style sheet, which can be the empty string.

In the following, the title is non-empty for the first style sheet, but is empty for the second and third style sheets.

<style title="papaya whip">
  body { background: #ffefd5; }
</style>

<style title="">
  body { background: orange; }
</style>

<style>
  body { background: brown; }
</style>

If this property is specified to an attribute of the owner node, the title must be set to the value of that attribute. Whenever the attribute is set, changed or removed, the title must be set to the new value of the attribute, or to the empty string if the attribute is absent.

Note: HTML only specifies title to be an attribute of the owner node if the node is in in a document tree.

alternate flag

Specified when created. Either set or unset. Unset by default.

The following CSS style sheets have their alternate flag set:

<?xml-stylesheet alternate="yes" title="x" href="data:text/css,…"?>

<link rel="alternate stylesheet" title="x" href="data:text/css,…">

disabled flag

Either set or unset. Unset by default.

Note: Even when unset it does not necessarily mean that the CSS style sheet is actually used for rendering.

CSS rules

The CSS rules associated with the CSS style sheet.

origin-clean flag

Specified when created. Either set or unset. If it is set, the API allows reading and modifying of the CSS rules.

constructed flag

Specified when created. Either set or unset. Unset by default. Signifies whether this stylesheet was created by invoking the IDL-defined constructor.

disallow modification flag

Either set or unset. Unset by default. If set, modification of the stylesheet’s rules is not allowed.

constructor document

Specified when created. The Document a constructed stylesheet is associated with. Null by default. Only non-null for stylesheets that have constructed flag set.

stylesheet base URL

The base URL to use when resolving relative URLs in the stylesheet. Null by default. Only non-null for stylesheets that have constructed flag set.

6.1.1. The StyleSheet Interface

The StyleSheet interface represents an abstract, base style sheet.

[Exposed=Window]
interface StyleSheet {
  readonly attribute CSSOMString type;
  readonly attribute USVString? href;
  readonly attribute (Element or ProcessingInstruction)? ownerNode;
  readonly attribute CSSStyleSheet? parentStyleSheet;
  readonly attribute DOMString? title;
  [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
  attribute boolean disabled;
};

The type attribute must return the type.

The href attribute must return the location.

The ownerNode attribute must return the owner node.

The parentStyleSheet attribute must return the parent CSS style sheet.

The title attribute must return the title or null if title is the empty string.

The media attribute must return the media.

The disabled attribute, on getting, must return true if the disabled flag is set, or false otherwise. On setting, the disabled attribute must set the disabled flag if the new value is true, or unset the disabled flag otherwise.

6.1.2. The CSSStyleSheet Interface

The CSSStyleSheet interface represents a CSS style sheet.

[Exposed=Window]
interface CSSStyleSheet : StyleSheet {
  constructor(optional CSSStyleSheetInit options = {});

  readonly attribute CSSRule? ownerRule;
  [SameObject] readonly attribute CSSRuleList cssRules;
  unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
  undefined deleteRule(unsigned long index);

  Promise<CSSStyleSheet> replace(USVString text);
  undefined replaceSync(USVString text);
};

dictionary CSSStyleSheetInit {
  DOMString? baseURL = null;
  (MediaList or DOMString) media = "";
  boolean disabled = false;
};

The ownerRule attribute must return the owner CSS rule. If a value other than null is ever returned, then that same value must always be returned on each get access.

The cssRules attribute must follow these steps:

1. If the origin-clean flag is unset, throw a SecurityError exception.
2. Return a read-only, live CSSRuleList object representing the CSS rules.

   Note: Even though the returned CSSRuleList object is read-only (from the perspective of client-authored script), it can nevertheless change over time due to its liveness status. For example, invoking the insertRule() or deleteRule() methods can result in mutations reflected in the returned object.

The insertRule(rule, index) method must run the following steps:

1. If the origin-clean flag is unset, throw a SecurityError exception.
2. If the disallow modification flag is set, throw a NotAllowedError DOMException.
3. Let parsed rule be the return value of invoking parse a rule with rule.
4. If parsed rule is a syntax error, throw a SyntaxError DOMException.
5. If parsed rule is an @import rule, and the constructed flag is set, throw a SyntaxError DOMException.
6. Return the result of invoking insert a CSS rule rule in the CSS rules at index.

The deleteRule(index) method must run the following steps:

1. If the origin-clean flag is unset, throw a SecurityError exception.
2. If the disallow modification flag is set, throw a NotAllowedError DOMException.
3. Remove a CSS rule in the CSS rules at index.

The replace(text) method must run the following steps:

1. Let promise be a promise.
2. If the constructed flag is not set, or the disallow modification flag is set, reject promise with a NotAllowedError DOMException and return promise.
3. Set the disallow modification flag.
4. In parallel, do these steps:
   1. Let rules be the result of running parse a stylesheet’s contents from text.
   2. If rules contains one or more @import rules, remove those rules from rules.
   3. Set sheet’s CSS rules to rules.
   4. Unset sheet’s disallow modification flag.
   5. Resolve promise with sheet.
5. Return promise.

The replaceSync(text) method must run the steps to synchronously replace the rules of a CSSStyleSheet on this CSSStyleSheet given text.

To synchronously replace the rules of a CSSStyleSheet on sheet given text, run these steps:

1. If the constructed flag is not set, or the disallow modification flag is set, throw a NotAllowedError DOMException.
2. Let rules be the result of running parse a stylesheet’s contents from text.
3. If rules contains one or more @import rules, remove those rules from rules.
4. Set sheet’s CSS rules to rules.

6.1.2.1. Deprecated CSSStyleSheet members

Note: These members are required for compatibility with existing sites.

partial interface CSSStyleSheet {
  [SameObject] readonly attribute CSSRuleList rules;
  long addRule(optional DOMString selector = "undefined", optional DOMString style = "undefined", optional unsigned long index);
  undefined removeRule(optional unsigned long index = 0);
};

The rules attribute must follow the same steps as cssRules, and return the same object cssRules would return.

The removeRule(index) method must run the same steps as deleteRule().

The addRule(selector, block, optionalIndex) method must run the following steps:

1. Let rule be an empty string.
2. Append selector to rule.
3. Append " { " to rule.
4. If block is not empty, append block, followed by a space, to rule.
5. Append "}" to rule
6. Let index be optionalIndex if provided, or the number of CSS rules in the stylesheet otherwise.
7. Call insertRule(), with rule and index as arguments.
8. Return -1.

Authors should not use these members and should instead use and teach the standard CSSStyleSheet interface defined earlier, which is consistent with CSSGroupingRule.

6.2. CSS Style Sheet Collections

Below various new concepts are defined that are associated with each DocumentOrShadowRoot object.

Each DocumentOrShadowRoot has an associated list of zero or more CSS style sheets, named the document or shadow root CSS style sheets. This is an ordered list that contains:

1. Any CSS style sheets created from HTTP Link headers, in header order
2. Any CSS style sheets associated with the DocumentOrShadowRoot, in tree order

Each DocumentOrShadowRoot has an associated list of zero or more CSS style sheets, named the final CSS style sheets. This is an ordered list that contains:

1. The document or shadow root CSS style sheets.
2. The contents of DocumentOrShadowRoot’s adoptedStyleSheets' backing list, in array order.

To create a CSS style sheet, run these steps:

1. Create a new CSS style sheet object and set its properties as specified.
2. Then run the add a CSS style sheet steps for the newly created CSS style sheet.

   If the origin-clean flag is unset, this can expose information from the user’s intranet.

To add a CSS style sheet, run these steps:

1. Add the CSS style sheet to the list of document or shadow root CSS style sheets at the appropriate location.

2. If the CSS style sheet’s owner node contributes a script-blocking style sheet, then user agents must append the owner node to its node document’s script-blocking style sheet set.

   The remainder of these steps deal with the disabled flag.

3. If the disabled flag is set, then return.
4. If the title is not the empty string, the alternate flag is unset, and preferred CSS style sheet set name is the empty string change the preferred CSS style sheet set name to the title.
5. If any of the following is true, then unset the disabled flag and return:
   • The title is the empty string.
   • The last CSS style sheet set name is null and the title is a case-sensitive match for the preferred CSS style sheet set name.
   • The title is a case-sensitive match for the last CSS style sheet set name.
6. Set the disabled flag.

To remove a CSS style sheet, run these steps:

1. Remove the CSS style sheet from the list of document or shadow root CSS style sheets.
2. Set the CSS style sheet’s parent CSS style sheet, owner node and owner CSS rule to null.

A persistent CSS style sheet is a CSS style sheet from the document or shadow root CSS style sheets whose title is the empty string and whose alternate flag is unset.

A CSS style sheet set is an ordered collection of one or more CSS style sheets from the document or shadow root CSS style sheets which have an identical title that is not the empty string.

A CSS style sheet set name is the title the CSS style sheet set has in common.

An enabled CSS style sheet set is a CSS style sheet set of which each CSS style sheet has its disabled flag unset.

To enable a CSS style sheet set with name name, run these steps:

1. If name is the empty string, set the disabled flag for each CSS style sheet that is in a CSS style sheet set and return.
2. Unset the disabled flag for each CSS style sheet in a CSS style sheet set whose CSS style sheet set name is a case-sensitive match for name and set it for all other CSS style sheets in a CSS style sheet set.

To select a CSS style sheet set with name name, run these steps:

1. enable a CSS style sheet set with name name.
2. Set last CSS style sheet set name to name.

A last CSS style sheet set name is a concept to determine what CSS style sheet set was last selected. Initially its value is null.

A preferred CSS style sheet set name is a concept to determine which CSS style sheets need to have their disabled flag unset. Initially its value is the empty string.

To change the preferred CSS style sheet set name with name name, run these steps:

1. Let current be the preferred CSS style sheet set name.
2. Set preferred CSS style sheet set name to name.
3. If name is not a case-sensitive match for current and last CSS style sheet set name is null enable a CSS style sheet set with name name.

6.2.1. The HTTP Default-Style Header

The HTTP Default-Style header can be used to set the preferred CSS style sheet set name influencing which CSS style sheet set is (initially) the enabled CSS style sheet set.

For each HTTP Default-Style header, in header order, the user agent must change the preferred CSS style sheet set name with name being the value of the header.

6.2.2. The StyleSheetList Interface

The StyleSheetList interface represents an ordered collection of CSS style sheets.

[Exposed=Window]
interface StyleSheetList {
  getter CSSStyleSheet? item(unsigned long index);
  readonly attribute unsigned long length;
};

The object’s supported property indices are the numbers in the range zero to one less than the number of CSS style sheets represented by the collection. If there are no such CSS style sheets, then there are no supported property indices.

The item(index) method must return the indexth CSS style sheet in the collection. If there is no indexth object in the collection, then the method must return null.

The length attribute must return the number of CSS style sheets represented by the collection.

6.2.3. Extensions to the DocumentOrShadowRoot Interface Mixin

partial interface mixin DocumentOrShadowRoot {
  [SameObject] readonly attribute StyleSheetList styleSheets;
  attribute ObservableArray<CSSStyleSheet> adoptedStyleSheets;
};

The styleSheets attribute must return a StyleSheetList collection representing the document or shadow root CSS style sheets.

The set an indexed value algorithm for adoptedStyleSheets, given value and index, is the following:

1. If value’s constructed flag is not set, or its constructor document is not equal to this DocumentOrShadowRoot’s node document, throw a "NotAllowedError" DOMException.

6.3. Style Sheet Association

This section defines the interface an owner node of a CSS style sheet has to implement and defines the requirements for xml-stylesheet processing instructions and HTTP Link headers when the link relation type is an ASCII case-insensitive match for "stylesheet".

6.3.1. Fetching CSS style sheets

6.3.2. The LinkStyle Interface

The associated CSS style sheet of a node is the CSS style sheet in the list of document or shadow root CSS style sheets of which the owner node is said node. This node must also implement the LinkStyle interface.

interface mixin LinkStyle {
  readonly attribute CSSStyleSheet? sheet;
};

The sheet attribute must return the associated CSS style sheet for the node or null if there is no associated CSS style sheet.

<style type="text/css">
  body { background:lime }
</style>

<style type="text/example-sheets">
  $(body).background := lime
</style>

Note: Whether or not the node refers to a style sheet is defined by the specification that defines the semantics of said node.

6.3.3. Requirements on specifications

Specifications introducing new ways of associating style sheets through the DOM should define which nodes implement the LinkStyle interface. When doing so, they must also define when a CSS style sheet is created.

6.3.4. Requirements on user agents Implementing the xml-stylesheet processing instruction

ProcessingInstruction includes LinkStyle;

The prolog refers to nodes that are children of the Document and are not following the Element child of the Document, if any.

6.3.5. Requirements on user agents Implementing the HTTP Link Header

For each HTTP Link header of which one of the link relation types is an ASCII case-insensitive match for "stylesheet" these steps must be run:

A style sheet referenced by a HTTP Link header using the rules in this section is said to be a style sheet that is blocking scripts if the style sheet was enabled when created, and the user agent hasn’t given up on that particular style sheet yet. A user agent may give up on such a style sheet at any time.

6.4. CSS Rules

A CSS rule is an abstract concept that denotes a rule as defined by the CSS specification. A CSS rule is represented as an object that implements a subclass of the CSSRule interface, and which has the following associated state items:

type

A non-negative integer associated with a particular type of rule. This item is initialized when a rule is created and cannot change.

text

A text representation of the rule suitable for direct use in a style sheet. This item is initialized when a rule is created and can be changed.

parent CSS rule

A reference to an enclosing CSS rule or null. If the rule has an enclosing rule when it is created, then this item is initialized to the enclosing rule; otherwise it is null. It can be changed to null.

parent CSS style sheet

A reference to a parent CSS style sheet or null. This item is initialized to reference an associated style sheet when the rule is created. It can be changed to null.

child CSS rules

A list of child CSS rules. The list can be mutated.

In addition to the above state, each CSS rule may be associated with other state in accordance with its type.

To parse a CSS rule from a string string, run the following steps:

1. Let rule be the return value of invoking parse a rule with string.
2. If rule is a syntax error, return rule.
3. Let parsed rule be the result of parsing rule according to the appropriate CSS specifications, dropping parts that are said to be ignored. If the whole rule is dropped, return a syntax error.
4. Return parsed rule.

To serialize a CSS rule, perform one of the following in accordance with the CSS rule’s type:

CSSStyleRule

Return the result of the following steps:

1. Let s initially be the result of performing serialize a group of selectors on the rule’s associated selectors, followed by the string " {", i.e., a single SPACE (U+0020), followed by LEFT CURLY BRACKET (U+007B).
2. Let decls be the result of performing serialize a CSS declaration block on the rule’s associated declarations, or null if there are no such declarations.
3. Let rules be the result of performing serialize a CSS rule on each rule in the rule’s cssRules list, or null if there are no such rules.
4. If decls and rules are both null, append " }" to s (i.e. a single SPACE (U+0020) followed by RIGHT CURLY BRACKET (U+007D)) and return s.
5. If rules is null:
   1. Append a single SPACE (U+0020) to s
   2. Append decls to s
   3. Append " }" to s (i.e. a single SPACE (U+0020) followed by RIGHT CURLY BRACKET (U+007D)).
   4. Return s.
6. Otherwise:
   1. If decls is not null, prepend it to rules.
   2. For each rule in rules:
      • If rule is the empty string, do nothing.
      • Otherwise:
        1. Append a newline followed by two spaces to s.
        2. Append rule to s.
   3. Append a newline followed by RIGHT CURLY BRACKET (U+007D) to s.
   4. Return s.

CSSImportRule

The result of concatenating the following:

1. The string "@import" followed by a single SPACE (U+0020).
2. The result of performing serialize a URL on the rule’s location.
3. If the rule’s associated media list is not empty, a single SPACE (U+0020) followed by the result of performing serialize a media query list on the media list.
4. The string ";", i.e., SEMICOLON (U+003B).

@import url("import.css");

@import url("print.css") print;

CSSMediaRule

The result of concatenating the following:

1. The string "@media", followed by a single SPACE (U+0020).
2. The result of performing serialize a media query list on rule’s media query list.
3. A single SPACE (U+0020), followed by the string "{", i.e., LEFT CURLY BRACKET (U+007B), followed by a newline.
4. The result of performing serialize a CSS rule on each rule in the rule’s cssRules list, filtering out empty strings, indenting each item with two spaces, all joined with newline.
5. A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)

CSSFontFaceRule

The result of concatenating the following:

1. The string "@font-face {".
2. If the font-family descriptor is present:
   1. A single SPACE (U+0020), followed by the string "font-family:", followed by a single SPACE (U+0020).
   2. The result of performing serialize a string on the rule’s font family name.
   3. The string ";", i.e., SEMICOLON (U+003B).
3. If the rule’s associated source list is not empty, follow these substeps:
   1. A single SPACE (U+0020), followed by the string "src:", followed by a single SPACE (U+0020).
   2. The result of invoking serialize a comma-separated list on performing serialize a URL or serialize a LOCAL for each source on the source list.
   3. The string ";", i.e., SEMICOLON (U+003B).
4. If rule’s associated unicode-range descriptor is present, a single SPACE (U+0020), followed by the string "unicode-range:", followed by a single SPACE (U+0020), followed by the result of performing serialize a <'unicode-range'>, followed by the string ";", i.e., SEMICOLON (U+003B).
5. If rule’s associated font-variant descriptor is present, a single SPACE (U+0020), followed by the string "font-variant:", followed by a single SPACE (U+0020), followed by the result of performing serialize a <'font-variant'>, followed by the string ";", i.e., SEMICOLON (U+003B).
6. If rule’s associated font-feature-settings descriptor is present, a single SPACE (U+0020), followed by the string "font-feature-settings:", followed by a single SPACE (U+0020), followed by the result of performing serialize a <'font-feature-settings'>, followed by the string ";", i.e., SEMICOLON (U+003B).
7. If rule’s associated font-stretch descriptor is present, a single SPACE (U+0020), followed by the string "font-stretch:", followed by a single SPACE (U+0020), followed by the result of performing serialize a <'font-stretch'>, followed by the string ";", i.e., SEMICOLON (U+003B).
8. If rule’s associated font-weight descriptor is present, a single SPACE (U+0020), followed by the string "font-weight:", followed by a single SPACE (U+0020), followed by the result of performing serialize a <'font-weight'>, followed by the string ";", i.e., SEMICOLON (U+003B).
9. If rule’s associated font-style descriptor is present, a single SPACE (U+0020), followed by the string "font-style:", followed by a single SPACE (U+0020), followed by the result of performing serialize a <'font-style'>, followed by the string ";", i.e., SEMICOLON (U+003B).
10. A single SPACE (U+0020), followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D).

Need to define how the CSSFontFaceRule descriptors' values are serialized.

CSSPageRule

Need to define how CSSPageRule is serialized.

CSSNamespaceRule

The literal string "@namespace", followed by a single SPACE (U+0020), followed by the serialization as an identifier of the prefix attribute (if any), followed by a single SPACE (U+0020) if there is a prefix, followed by the serialization as URL of the namespaceURI attribute, followed the character ";" (U+003B).

CSSKeyframesRule

The result of concatenating the following:

1. The literal string "@keyframes", followed by a single SPACE (U+0020).
2. The serialization of the name attribute. If the attribute is a CSS wide keyword, or the value default, or the value none, then it is serialized as a string. Otherwise, it is serialized as an identifier.
3. The string " { ", i.e., a single SPACE (U+0020), followed by LEFT CURLY BRACKET (U+007B), followed by a single SPACE (U+0020).
4. The result of performing serialize a CSS rule on each rule in the rule’s cssRules list, separated by a newline and indented by two spaces.
5. A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)

CSSKeyframeRule

The result of concatenating the following:

1. The keyText.
2. The string " { ", i.e., a single SPACE (U+0020), followed by LEFT CURLY BRACKET (U+007B), followed by a single SPACE (U+0020).
3. The result of performing serialize a CSS declaration block on the rule’s associated declarations.
4. If the rule is associated with one or more declarations, the string " ", i.e., a single SPACE (U+0020).
5. The string "}", RIGHT CURLY BRACKET (U+007D).

The "indented by two spaces" bit matches browsers, but needs work, see #5494

To insert a CSS rule rule in a CSS rule list list at index index, with a flag nested, follow these steps:

1. Set length to the number of items in list.
2. If index is greater than length, then throw an IndexSizeError exception.
3. Set new rule to the results of performing parse a CSS rule on argument rule.
4. If new rule is a syntax error, and nested is set, perform the following substeps:
   • Set declarations to the results of performing parse a CSS declaration block, on argument rule.
   • If declarations is empty, throw a SyntaxError exception.
   • Otherwise, set new rule to a new nested declarations rule with declarations as it contents.
5. If new rule is a syntax error, throw a SyntaxError exception.
6. If new rule cannot be inserted into list at the zero-indexed position index due to constraints specified by CSS, then throw a HierarchyRequestError exception. [CSS21]

   Note: For example, a CSS style sheet cannot contain an @import at-rule after a style rule.

7. If new rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.
8. Insert new rule into list at the zero-indexed position index.
9. Return index.

To remove a CSS rule from a CSS rule list list at index index, follow these steps:

1. Set length to the number of items in list.
2. If index is greater than or equal to length, then throw an IndexSizeError exception.
3. Set old rule to the indexth item in list.
4. If old rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.
5. Remove rule old rule from list at the zero-indexed position index.
6. Set old rule’s parent CSS rule and parent CSS style sheet to null.

6.4.1. The CSSRuleList Interface

The CSSRuleList interface represents an ordered collection of CSS rules.

[Exposed=Window]
interface CSSRuleList {
  getter CSSRule? item(unsigned long index);
  readonly attribute unsigned long length;
};

The object’s supported property indices are the numbers in the range zero to one less than the number of CSSRule objects represented by the collection. If there are no such CSSRule objects, then there are no supported property indices.

The item(index) method must return the indexth CSSRule object in the collection. If there is no indexth object in the collection, then the method must return null.

The length attribute must return the number of CSSRule objects represented by the collection.

6.4.2. The CSSRule Interface

The CSSRule interface represents an abstract, base CSS rule. Each distinct CSS rule type is represented by a distinct interface that inherits from this interface.

[Exposed=Window]
interface CSSRule {
  attribute CSSOMString cssText;
  readonly attribute CSSRule? parentRule;
  readonly attribute CSSStyleSheet? parentStyleSheet;

  // the following attribute and constants are historical
  readonly attribute unsigned short type;
  const unsigned short STYLE_RULE = 1;
  const unsigned short CHARSET_RULE = 2;
  const unsigned short IMPORT_RULE = 3;
  const unsigned short MEDIA_RULE = 4;
  const unsigned short FONT_FACE_RULE = 5;
  const unsigned short PAGE_RULE = 6;
  const unsigned short MARGIN_RULE = 9;
  const unsigned short NAMESPACE_RULE = 10;
};

The cssText attribute must return a serialization of the CSS rule. On setting the cssText attribute must do nothing.

The parentRule attribute must return the parent CSS rule.

Note: For example, @media can enclose a rule, in which case parentRule would be non-null; in cases where there is no enclosing rule, parentRule will be null.

The parentStyleSheet attribute must return the parent CSS style sheet.

Note: The only circumstance where null is returned when a rule has been removed.

Note: Removing a Node that implements the LinkStyle interface from a Document instance does not (by itself) cause the CSSStyleSheet referenced by a CSSRule to be unreachable.

The type attribute is deprecated. It must return an integer, as follows:

If the object is a CSSStyleRule

Return 1.

If the object is a CSSImportRule

Return 3.

If the object is a CSSMediaRule

Return 4.

If the object is a CSSFontFaceRule

Return 5.

If the object is a CSSPageRule

Return 6.

If the object is a CSSKeyframesRule

Return 7.

If the object is a CSSKeyframeRule

Return 8.

If the object is a CSSMarginRule

Return 9.

If the object is a CSSNamespaceRule

Return 10.

If the object is a CSSCounterStyleRule

Return 11.

If the object is a CSSSupportsRule

Return 12.

If the object is a CSSFontFeatureValuesRule

Return 14.

Otherwise

Return 0.

Note: The practice of using an integer enumeration and several constants to identify the integers is a legacy design practice that is no longer used in Web APIs. Instead, to tell what type of rule a given object is, it is recommended to check rule.constructor.name, which will return a string like "CSSStyleRule". This enumeration is thus frozen in its current state, and no new new values will be added to reflect additional at-rules; all at-rules beyond the ones listed above will return 0.

6.4.3. The CSSStyleRule Interface

The CSSStyleRule interface represents a style rule.

[Exposed=Window]
interface CSSStyleRule : CSSGroupingRule {
  attribute CSSOMString selectorText;
  [SameObject, PutForwards=cssText] readonly attribute CSSStyleProperties style;
};

The selectorText attribute, on getting, must return the result of serializing the rule’s associated selector list. On setting the selectorText attribute these steps must be run:

1. Run the parse a group of selectors algorithm on the given value.
2. If the algorithm returns a non-null value replace the associated selector list with the returned value.
3. Otherwise, if the algorithm returns a null value, do nothing.

The style attribute must return a CSSStyleProperties object for the style rule, with the following properties:

computed flag

Unset.

readonly flag

Unset.

declarations

The declared declarations in the rule, in specified order.

parent CSS rule

this.

owner node

Null.

The specified order for declarations is the same as specified, but with shorthand properties expanded into their longhand properties, in canonical order. If a property is specified more than once (after shorthand expansion), only the one with greatest cascading order must be represented, at the same relative position as it was specified. [CSS3CASCADE]

6.4.4. The CSSImportRule Interface

The CSSImportRule interface represents an @import at-rule.

[Exposed=Window]
interface CSSImportRule : CSSRule {
  readonly attribute USVString href;
  [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
  [SameObject] readonly attribute CSSStyleSheet? styleSheet;
  readonly attribute CSSOMString? layerName;
  readonly attribute CSSOMString? supportsText;
};

The href attribute must return the URL specified by the @import at-rule.

Note: To get the resolved URL use the href attribute of the associated CSS style sheet.

The media attribute must return the value of the media attribute of the associated CSS style sheet.

The styleSheet attribute must return the associated CSS style sheet, if any, or null otherwise.

The layerName attribute must return the layer name declared in the at-rule itself, or an empty string if the layer is anonymous, or null if the at-rule does not declare a layer.

The supportsText attribute must return the <supports-condition> declared in the at-rule itself, or null if the at-rule does not declare a supports condition.

Note: An @import at-rule might not have an associated CSS style sheet (e.g., if it has a non-matching supports() condition).

6.4.5. The CSSGroupingRule Interface

The CSSGroupingRule interface represents an at-rule that contains other rules nested inside itself.

[Exposed=Window]
interface CSSGroupingRule : CSSRule {
  [SameObject] readonly attribute CSSRuleList cssRules;
  unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
  undefined deleteRule(unsigned long index);
};

The cssRules attribute must return a CSSRuleList object for the child CSS rules.

The insertRule(rule, index) method must return the result of invoking insert a CSS rule rule into the child CSS rules at index, with the nested flag set.

The deleteRule(index) method must remove a CSS rule from the child CSS rules at index.

6.4.6. The CSSMediaRule Interface

The CSSMediaRule interface is defined in CSS Conditional Rules. [CSS3-CONDITIONAL]

6.4.7. The CSSPageRule Interface

The CSSPageRule interface represents an @page at-rule.

Need to define the rules for parse a list of CSS page selectors and serialize a list of CSS page selectors.

[Exposed=Window]
interface CSSPageDescriptors : CSSStyleDeclaration {
  attribute [LegacyNullToEmptyString] CSSOMString margin;
  attribute [LegacyNullToEmptyString] CSSOMString marginTop;
  attribute [LegacyNullToEmptyString] CSSOMString marginRight;
  attribute [LegacyNullToEmptyString] CSSOMString marginBottom;
  attribute [LegacyNullToEmptyString] CSSOMString marginLeft;
  attribute [LegacyNullToEmptyString] CSSOMString margin-top;
  attribute [LegacyNullToEmptyString] CSSOMString margin-right;
  attribute [LegacyNullToEmptyString] CSSOMString margin-bottom;
  attribute [LegacyNullToEmptyString] CSSOMString margin-left;
  attribute [LegacyNullToEmptyString] CSSOMString size;
  attribute [LegacyNullToEmptyString] CSSOMString pageOrientation;
  attribute [LegacyNullToEmptyString] CSSOMString page-orientation;
  attribute [LegacyNullToEmptyString] CSSOMString marks;
  attribute [LegacyNullToEmptyString] CSSOMString bleed;
};

[Exposed=Window]
interface CSSPageRule : CSSGroupingRule {
           attribute CSSOMString selectorText;
  [SameObject, PutForwards=cssText] readonly attribute CSSPageDescriptors style;
};

The selectorText attribute, on getting, must return the result of serializing the associated selector list. On setting the selectorText attribute these steps must be run:

1. Run the parse a list of CSS page selectors algorithm on the given value.
2. If the algorithm returns a non-null value replace the associated selector list with the returned value.
3. Otherwise, if the algorithm returns a null value, do nothing.

The style attribute must return a CSSPageDescriptors object for the @page at-rule, with the following properties:

computed flag

Unset.

readonly flag

Unset.

declarations

The declared descriptors in the rule, in specified order.

parent CSS rule

this.

owner node

Null.

6.4.8. The CSSMarginRule Interface

The CSSMarginRule interface represents a margin at-rule (e.g. @top-left) in an @page at-rule. [CSS3PAGE]

[Exposed=Window]
interface CSSMarginRule : CSSRule {
  readonly attribute CSSOMString name;
  [SameObject, PutForwards=cssText] readonly attribute CSSMarginDescriptors style;
};

The name attribute must return the name of the margin at-rule. The @ character is not included in the name. [CSS3SYN]

The style attribute must return a CSSMarginDescriptors object for the margin at-rule, with the following properties:

computed flag

Unset.

readonly flag

Unset.

declarations

The declared declarations in the rule, in specified order.

parent CSS rule

this.

owner node

Null.

6.4.9. The CSSNamespaceRule Interface

The CSSNamespaceRule interface represents an @namespace at-rule.

[Exposed=Window]
interface CSSNamespaceRule : CSSRule {
  readonly attribute CSSOMString namespaceURI;
  readonly attribute CSSOMString prefix;
};

The namespaceURI attribute must return the namespace of the @namespace at-rule.

The prefix attribute must return the prefix of the @namespace at-rule or the empty string if there is no prefix.

6.5. CSS Declarations

A CSS declaration is an abstract concept that is not exposed as an object in the DOM. A CSS declaration has the following associated properties:

property name

The property name of the declaration.

value

The value of the declaration represented as a list of component values.

important flag

Either set or unset. Can be changed.

case-sensitive flag

Set if the property name is defined to be case-sensitive according to its specification, otherwise unset.

6.6. CSS Declaration Blocks

A CSS declaration block is an ordered collection of CSS properties with their associated values, also named CSS declarations. In the DOM a CSS declaration block is a CSSStyleDeclaration object. A CSS declaration block has the following associated properties:

computed flag

Set if the object is a computed style declaration, rather than a specified style. Unless otherwise stated it is unset.

readonly flag

Set if the object is not modifiable.

declarations

The CSS declarations associated with the object.

parent CSS rule

The CSS rule that the CSS declaration block is associated with, if any, or null otherwise.

owner node

The Element that the CSS declaration block is associated with, if any, or null otherwise.

updating flag

Unset by default. Set when the CSS declaration block is updating the owner node’s style attribute.

To parse a CSS declaration block from a string string, follow these steps:

1. Let declarations be the returned declarations from invoking parse a list of declarations with string.
2. Let parsed declarations be a new empty list.
3. For each item declaration in declarations, follow these substeps:
   1. Let parsed declaration be the result of parsing declaration according to the appropriate CSS specifications, dropping parts that are said to be ignored. If the whole declaration is dropped, let parsed declaration be null.
   2. If parsed declaration is not null, append it to parsed declarations.
4. Return parsed declarations.

To serialize a CSS declaration with property name property, value value and optionally an important flag set, follow these steps:

1. Let s be the empty string.
2. Append property to s.
3. Append ": " (U+003A U+0020) to s.
4. If value contains any non-whitespace characters, append value to s.
5. If the important flag is set, append " !important" (U+0020 U+0021 U+0069 U+006D U+0070 U+006F U+0072 U+0074 U+0061 U+006E U+0074) to s.
6. Append ";" (U+003B) to s.
7. Return s.

Note: The serialization of an empty CSS declaration block is the empty string.

Note: The serialization of a non-empty CSS declaration block does not include any surrounding whitespace, i.e., no whitespace appears before the first property name and no whitespace appears after the final semicolon delimiter that follows the last property value.

A CSS declaration block has these attribute change steps for its owner node with localName, value, and namespace:

1. If the computed flag is set, then return.
2. If the updating flag is set, then return.
3. If localName is not "style", or namespace is not null, then return.
4. If value is null, empty the declarations.
5. Otherwise, let the declarations be the result of parse a CSS declaration block from a string value.

When a CSS declaration block object is created, then:

1. Let owner node be the owner node.
2. If owner node is null, or the computed flag is set, then return.
3. Let value be the result of getting an attribute given null, "style", and owner node.
4. If value is not null, let the declarations be the result of parse a CSS declaration block from a string value.

To update style attribute for declaration block means to run the steps below:

1. Assert: declaration block’s computed flag is unset.
2. Let owner node be declaration block’s owner node.
3. If owner node is null, then return.
4. Set declaration block’s updating flag.
5. Set an attribute value for owner node using "style" and the result of serializing declaration block.
6. Unset declaration block’s updating flag.

6.6.1. The CSSStyleDeclaration Interface

The CSSStyleDeclaration interface represents a CSS declaration block, including its underlying state, where this underlying state depends upon the source of the CSSStyleDeclaration instance.

[Exposed=Window]
interface CSSStyleDeclaration {
  [CEReactions] attribute CSSOMString cssText;
  readonly attribute unsigned long length;
  getter CSSOMString item(unsigned long index);
  CSSOMString getPropertyValue(CSSOMString property);
  CSSOMString getPropertyPriority(CSSOMString property);
  [CEReactions] undefined setProperty(CSSOMString property, [LegacyNullToEmptyString] CSSOMString value, optional [LegacyNullToEmptyString] CSSOMString priority = "");
  [CEReactions] CSSOMString removeProperty(CSSOMString property);
  readonly attribute CSSRule? parentRule;
};

[Exposed=Window]
interface CSSStyleProperties : CSSStyleDeclaration {
  [CEReactions] attribute [LegacyNullToEmptyString] CSSOMString cssFloat;
};

The object’s supported property indices are the numbers in the range zero to one less than the number of CSS declarations in the declarations. If there are no such CSS declarations, then there are no supported property indices.

Getting the cssText attribute must run these steps:

1. If the computed flag is set, then return the empty string.

2. Return the result of serializing the declarations.

Setting the cssText attribute must run these steps:

1. If the readonly flag is set, then throw a NoModificationAllowedError exception.
2. Empty the declarations.
3. Parse the given value and, if the return value is not the empty list, insert the items in the list into the declarations, in specified order.
4. Update style attribute for the CSS declaration block.

The length attribute must return the number of CSS declarations in the declarations.

The item(index) method must return the property name of the CSS declaration at position index. If there is no indexth object in the collection, then the method must return the empty string.

The getPropertyValue(property) method must run these steps:

1. If property is not a custom property, follow these substeps:
   1. Let property be property converted to ASCII lowercase.
   2. If property is a shorthand property, then follow these substeps:
      1. Let list be a new empty array.
      2. For each longhand property longhand that property maps to, in canonical order, follow these substeps:
         1. If longhand is a case-sensitive match for a property name of a CSS declaration in the declarations, let declaration be that CSS declaration, or null otherwise.
         2. If declaration is null, then return the empty string.
         3. Append the declaration to list.
      3. If important flags of all declarations in list are same, then return the serialization of list.
      4. Return the empty string.
2. If property is a case-sensitive match for a property name of a CSS declaration in the declarations, then return the result of invoking serialize a CSS value of that declaration.
3. Return the empty string.

The getPropertyPriority(property) method must run these steps:

1. If property is not a custom property, follow these substeps:
   1. Let property be property converted to ASCII lowercase.
   2. If property is a shorthand property, follow these substeps:
      1. Let list be a new array.
      2. For each longhand property longhand that property maps to, append the result of invoking getPropertyPriority() with longhand as argument to list.
      3. If all items in list are the string "important", then return the string "important".
2. If property is a case-sensitive match for a property name of a CSS declaration in the declarations that has the important flag set, return the string "important".
3. Return the empty string.

The setProperty(property, value, priority) method must run these steps:

1. If the readonly flag is set, then throw a NoModificationAllowedError exception.
2. If property is not a custom property, follow these substeps:
   1. Let property be property converted to ASCII lowercase.
   2. If property is not a case-sensitive match for a supported CSS property, then return.
3. If value is the empty string, invoke removeProperty() with property as argument and return.
4. If priority is not the empty string and is not an ASCII case-insensitive match for the string "important", then return.
5. Let component value list be the result of parsing value for property property.

   Note: value can not include "!important".

6. If component value list is null, then return.
7. Let updated be false.
8. If property is a shorthand property, then for each longhand property longhand that property maps to, in canonical order, follow these substeps:
   1. Let longhand result be the result of set the CSS declaration longhand with the appropriate value(s) from component value list, with the important flag set if priority is not the empty string, and unset otherwise, and with the list of declarations being the declarations.
   2. If longhand result is true, let updated be true.
9. Otherwise, let updated be the result of set the CSS declaration property with value component value list, with the important flag set if priority is not the empty string, and unset otherwise, and with the list of declarations being the declarations.
10. If updated is true, update style attribute for the CSS declaration block.

To set a CSS declaration property with a value component value list and optionally with an important flag set, in a list of declarations declarations, the user agent must ensure the following constraints hold after its steps:

• Exactly one CSS declaration whose property name is a case-sensitive match of property must exist in declarations. Such declaration is referenced as the target declaration below.
• The target declaration must have value being component value list, and target declaration’s important flag must be set if important flag is set, and unset otherwise.
• Any CSS declaration which is not the target declaration must not be changed, inserted, or removed from declarations.
• If there are CSS declarations in declarations whose property name is in the same logical property group as property, but has a different mapping logic, target declaration must be at an index after all of those CSS declarations.
• The steps must return true if the serialization of declarations was changed as result of the steps. It may return false otherwise.

Should we add something like "Any observable side effect must not be made outside declarations"? The current constraints sound like a hole for undefined behavior.

Note: The steps of set a CSS declaration are not defined in this level of CSSOM. User agents may use different algorithms as long as the constraints above hold.

The removeProperty(property) method must run these steps:

1. If the readonly flag is set, then throw a NoModificationAllowedError exception.
2. If property is not a custom property, let property be property converted to ASCII lowercase.
3. Let value be the return value of invoking getPropertyValue() with property as argument.
4. Let removed be false.
5. If property is a shorthand property, for each longhand property longhand that property maps to:
   1. If longhand is not a property name of a CSS declaration in the declarations, continue.
   2. Remove that CSS declaration and let removed be true.
6. Otherwise, if property is a case-sensitive match for a property name of a CSS declaration in the declarations, remove that CSS declaration and let removed be true.
7. If removed is true, Update style attribute for the CSS declaration block.
8. Return value.

The parentRule attribute must return the parent CSS rule.

The cssFloat attribute, on getting, must return the result of invoking getPropertyValue() with float as argument. On setting, the attribute must invoke setProperty() with float as first argument, as second argument the given value, and no third argument. Any exceptions thrown must be re-thrown.

For each CSS property property that is a supported CSS property, the following partial interface applies where camel-cased attribute is obtained by running the CSS property to IDL attribute algorithm for property.

partial interface CSSStyleProperties {
  [CEReactions] attribute [LegacyNullToEmptyString] CSSOMString _camel_cased_attribute;
};

The camel-cased attribute attribute, on getting, must return the result of invoking getPropertyValue() with the argument being the result of running the IDL attribute to CSS property algorithm for camel-cased attribute.

Setting the camel-cased attribute attribute must invoke setProperty() with the first argument being the result of running the IDL attribute to CSS property algorithm for camel-cased attribute, as second argument the given value, and no third argument. Any exceptions thrown must be re-thrown.

For each CSS property property that is a supported CSS property and that begins with the string -webkit-, the following partial interface applies where webkit-cased attribute is obtained by running the CSS property to IDL attribute algorithm for property, with the lowercase first flag set.

partial interface CSSStyleProperties {
  [CEReactions] attribute [LegacyNullToEmptyString] CSSOMString _webkit_cased_attribute;
};

The webkit-cased attribute attribute, on getting, must return the result of invoking getPropertyValue() with the argument being the result of running the IDL attribute to CSS property algorithm for webkit-cased attribute, with the dash prefix flag set.

Setting the webkit-cased attribute attribute must invoke setProperty() with the first argument being the result of running the IDL attribute to CSS property algorithm for webkit-cased attribute, with the dash prefix flag set, as second argument the given value, and no third argument. Any exceptions thrown must be re-thrown.

For each CSS property property that is a supported CSS property, except for properties that have no "-" (U+002D) in the property name, the following partial interface applies where dashed attribute is property.

partial interface CSSStyleProperties {
  [CEReactions] attribute [LegacyNullToEmptyString] CSSOMString _dashed_attribute;
};

The dashed attribute attribute, on getting, must return the result of invoking getPropertyValue() with the argument being dashed attribute.

Setting the dashed attribute attribute must invoke setProperty() with the first argument being dashed attribute, as second argument the given value, and no third argument. Any exceptions thrown must be re-thrown.

element.style['font-size'];

The CSS property to IDL attribute algorithm for property, optionally with a lowercase first flag set, is as follows:

1. Let output be the empty string.
2. Let uppercase next be unset.
3. If the lowercase first flag is set, remove the first character from property.
4. For each character c in property:
   1. If c is "-" (U+002D), let uppercase next be set.
   2. Otherwise, if uppercase next is set, let uppercase next be unset and append c converted to ASCII uppercase to output.
   3. Otherwise, append c to output.
5. Return output.

The IDL attribute to CSS property algorithm for attribute, optionally with a dash prefix flag set, is as follows:

1. Let output be the empty string.
2. If the dash prefix flag is set, append "-" (U+002D) to output.
3. For each character c in attribute:
   1. If c is in the range U+0041 to U+005A (ASCII uppercase), append "-" (U+002D) followed by c converted to ASCII lowercase to output.
   2. Otherwise, append c to output.
4. Return output.

6.7. CSS Values

6.7.1. Parsing CSS Values

To parse a CSS value value for a given property means to follow these steps:

1. Let list be the value returned by invoking parse a list of component values from value.
2. Match list against the grammar for the property property in the CSS specification.
3. If the above step failed, return null.
4. Return list.

Note: "!important" declarations are not part of the property value space and will therefore cause parse a CSS value to return null.

6.7.2. Serializing CSS Values

To serialize a CSS value of a CSS declaration declaration or a list of longhand CSS declarations list, follow these rules:

1. If this algorithm is invoked with a list list:

   1. Let shorthand be the first shorthand property, in preferred shorthand order, that exactly maps to all of the longhand properties in list.

   2. If there is no such shorthand or shorthand cannot exactly represent the values of all the properties in list, return the empty string.

   3. Otherwise, serialize a CSS value from a hypothetical declaration of the property shorthand with its value representing the combined values of the declarations in list.

2. Represent the value of the declaration as a list of CSS component values components that, when parsed according to the property’s grammar, would represent that value. Additionally, unless otherwise specified:

   • If certain component values can appear in any order without changing the meaning of the value (a pattern typically represented by a double bar || in the value syntax), reorder the component values to use the canonical order of component values as given in the property definition table.

   • If component values can be omitted or replaced with a shorter representation without changing the meaning of the value, omit/replace them.

   • If either of the above syntactic translations would be less backwards-compatible, do not perform them.

   Note: The rules described here outline the general principles of serialization. For legacy reasons, some properties serialize in a different manner, which is intentionally undefined here due to lack of resources. Please consult that property’s specification and/or your local reverse-engineer for details.

3. Remove any <whitespace-token>s from components.

4. Replace each component value in components with the result of invoking serialize a CSS component value.

5. Join the items of components into a single string, inserting " " (U+0020 SPACE) between each pair of items unless the second item is a "," (U+002C COMMA) Return the result.

To serialize a CSS component value depends on the component, as follows:

keyword

The keyword converted to ASCII lowercase.

<angle>

The <number> component serialized as per <number> followed by the unit in canonical form as defined in its respective specification.

Probably should distinguish between declared and computed / resolved values.

<color>

If <color> is a component of a resolved value, see CSS Color 4 §  15. Resolving <color> Values.

If <color> is a component of a computed value, see CSS Color 4 §  16. Serializing <color> Values.

If <color> is a component of a declared value, then

for sRGB values, see CSS Color 4 § 15.1 Resolving sRGB values. For other color functions, see CSS Color 4 §  15. Resolving <color> Values.

<alpha-value>

See CSS Color 4 § 16.1 Serializing alpha values.

<counter>

The return value of the following algorithm:

1. Let s be the empty string.
2. If <counter> has three CSS component values append the string "counters(" to s.
3. If <counter> has two CSS component values append the string "counter(" to s.
4. Let list be a list of CSS component values belonging to <counter>, omitting the last CSS component value if it is "decimal".
5. Let each item in list be the result of invoking serialize a CSS component value on that item.
6. Append the result of invoking serialize a comma-separated list on list to s.
7. Append ")" (U+0029) to s.
8. Return s.

<frequency>

The <number> component serialized as per <number> followed by the unit in its canonical form as defined in its respective specification.

Probably should distinguish between declared and computed / resolved values.

<identifier>

The identifier serialized as an identifier.

<integer>

A base-ten integer using digits 0-9 (U+0030 to U+0039) in the shortest form possible, preceded by "-" (U+002D) if it is negative.

<length>

The <number> component serialized as per <number> followed by the unit in its canonical form as defined in its respective specification.

Probably should distinguish between declared and computed / resolved values.

<number>

A base-ten number using digits 0-9 (U+0030 to U+0039) in the shortest form possible, using "." to separate decimals (if any), rounding the value if necessary to not produce more than 6 decimals, preceded by "-" (U+002D) if it is negative.

Note: scientific notation is not used.

<percentage>

The <number> component serialized as per <number> followed by the literal string "%" (U+0025).

<resolution>

The resolution in dots per CSS pixel serialized as per <number> followed by the literal string "dppx".

<ratio>

The numerator serialized as per <number> followed by the literal string " / ", followed by the denominator serialized as per <number>.

<shape>

The return value of the following algorithm:

1. Let s be the string "rect(".
2. Let list be a list of the CSS component values belonging to <shape>.
3. Let each item in list be the result of invoking serialize a CSS component value of that item.
4. Append the result of invoking serialize a comma-separated list on list to s.
5. Append ")" (U+0029) to s.
6. Return s.

<string>

<font-family-name>

The string serialized as a string.

<time>

The time in seconds serialized as per <number> followed by the literal string "s".

<url>

The absolute-URL string serialized as URL.

This should differentiate declared and computed <url> values, see #3195.

<absolute-size>, <border-width>, <border-style>, <bottom>, <generic-font-family>, <generic-voice>, <left>, <margin-width>, <padding-width>, <relative-size>, <right>, and <top>, are considered macros by this specification. They all represent instances of components outlined above.

One idea is that we can remove this section somewhere in the CSS3/CSS4 timeline by moving the above definitions to the drafts that define the CSS components.

6.7.2.1. Examples

Here are some examples of before and after results on declared values. The before column could be what the author wrote in a style sheet, while the after column shows what querying the DOM would return.

7. DOM Access to CSS Declaration Blocks

7.1. The ElementCSSInlineStyle Mixin

The ElementCSSInlineStyle mixin provides access to inline style properties of an element.

interface mixin ElementCSSInlineStyle {
  [SameObject, PutForwards=cssText] readonly attribute CSSStyleProperties style;
};

The style attribute must return a CSSStyleProperties object whose readonly flag is unset, whose parent CSS rule is null, and whose owner node is this.

If the user agent supports HTML, the following IDL applies: [HTML]

HTMLElement includes ElementCSSInlineStyle;

If the user agent supports SVG, the following IDL applies: [SVG11]

SVGElement includes ElementCSSInlineStyle;

If the user agent supports MathML, the following IDL applies: [MathML-Core]

MathMLElement includes ElementCSSInlineStyle;

7.2. Extensions to the Window Interface

partial interface Window {
  [NewObject] CSSStyleProperties getComputedStyle(Element elt, optional CSSOMString? pseudoElt);
};

The getComputedStyle(elt, pseudoElt) method must run these steps:

1. Let doc be elt’s node document.
2. Let obj be elt.
3. If pseudoElt is provided, is not the empty string, and starts with a colon, then:
   1. Parse pseudoElt as a <pseudo-element-selector>, and let type be the result.
   2. If type is failure, or is a ::slotted() or ::part() pseudo-element, let obj be null.
   3. Otherwise let obj be the given pseudo-element of elt.

   Note: CSS2 pseudo-elements should match both the double and single-colon versions. That is, both :before and ::before should match above.

4. Let decls be an empty list of CSS declarations.
5. If obj is not null, and elt is connected, part of the flat tree, and its shadow-including root has a browsing context which either doesn’t have a browsing context container, or whose browsing context container is being rendered, set decls to a list of all longhand properties that are supported CSS properties, in lexicographical order, with the value being the resolved value computed for obj using the style rules associated with doc. Additionally, append to decls all the custom properties whose computed value for obj is not the guaranteed-invalid value.

   There are UAs that handle shorthands, and all UAs handle shorthands that used to be longhands like overflow, see #2529.

   Order of custom properties is currently undefined, see #4947.

6. Return a live CSSStyleProperties object with the following properties:

   computed flag

   Set.

   readonly flag

   Set.

   declarations

   decls.

   parent CSS rule

   Null.

   owner node

   obj.

The getComputedStyle() method exposes information from CSS style sheets with the origin-clean flag unset.

8. Utility APIs

8.1. The CSS.escape() Method

The CSS namespace holds useful CSS-related functions that do not belong elsewhere.

[Exposed=Window]
namespace CSS {
  CSSOMString escape(CSSOMString ident);
};

This was previously specified as an IDL interface that only held static methods. Switching to an IDL namespace is *nearly* identical, so it’s expected that there won’t be any compat concerns. If any are discovered, please report so we can consider reverting this change.

The escape(ident) operation must return the result of invoking serialize an identifier of ident.

var element = document.querySelector('#' + CSS.escape(id) + ' > img');

var element = document.querySelector('a[href="#' + CSS.escape(fragment) + '"]');

Specifications that define operations on the CSS namespace and want to store some state should store the state on the current global object’s associated Document.

9. Resolved Values

getComputedStyle() was historically defined to return the "computed value" of an element or pseudo-element. However, the concept of "computed value" changed between revisions of CSS while the implementation of getComputedStyle() had to remain the same for compatibility with deployed scripts. To address this issue this specification introduces the concept of a resolved value.

The resolved value for a given longhand property can be determined as follows:

background-color

border-block-end-color

border-block-start-color

border-bottom-color

border-inline-end-color

border-inline-start-color

border-left-color

border-right-color

border-top-color

box-shadow

caret-color

color

outline-color

A resolved value special case property like color defined in another specification

The resolved value is the used value.

line-height

The resolved value is normal if the computed value is normal, or the used value otherwise.

block-size

height

inline-size

margin-block-end

margin-block-start

margin-bottom

margin-inline-end

margin-inline-start

margin-left

margin-right

margin-top

padding-block-end

padding-block-start

padding-bottom

padding-inline-end

padding-inline-start

padding-left

padding-right

padding-top

width

A resolved value special case property like height defined in another specification

If the property applies to the element or pseudo-element and the resolved value of the display property is not none or contents, then the resolved value is the used value. Otherwise the resolved value is the computed value.

bottom

left

inset-block-end

inset-block-start

inset-inline-end

inset-inline-start

right

top

A resolved value special case property like top defined in another specification

If the property applies to a positioned element and the resolved value of the display property is not none or contents, and the property is not over-constrained, then the resolved value is the used value. Otherwise the resolved value is the computed value.

A resolved value special case property defined in another specification

As defined in the relevant specification.

Any other property

The resolved value is the computed value.

10. IANA Considerations

10.1. Default-Style

This section describes a header field for registration in the Permanent Message Header Field Registry.

Header field name

Default-Style

Applicable protocol

http

Status

standard

Author/Change controller

W3C

Specification document(s)

This document is the relevant specification.

Related information

None.

11. Change History

This section documents some of the changes between publications of this specification. This section is not exhaustive. Bug fixes and editorial changes are generally not listed.

11.1. Changes From 17 March 2016

• Serialization of <resolution> is changed.

• [CEReactions] IDL extended attributes are added.

• Resolved value for logical properties are added.

• getComputedStyle for display: contents is changed.

• MediaList.item now returns serialization.

• MediaList.item does not serialize shorthand if importance differs.

• Other specifications are allowed to specify resolved value.

• index argument in insertRule is now optional.

• href attribute of Stylesheet and CSSImportRule now uses USVString.

• CSSOMString is introduced.

• Serialization of CSSMediaRule and CSSFontFaceRule is added.

• Updating flag is added to CSS declaration block to avoid serialize-and-reparse on style attribute.

• Serialization of a declaration value is now properly defined.

• getComputedStyle now returns the style rules of the node’s document.

• A TypeError is thrown when the pseudo-element passed to getComputedStyle is unknown or ::slotted().

• CSS is switched from interface to namespace.

• setPropertyValue and setPropertyPriority are removed from CSSStyleDeclaration due to lack of interest from implementations.

• The styleSheets IDL attribute is moved from Document to DocumentOrShadowRoot.

• LinkStyle.sheet now returns CSSStyleSheet instead of StyleSheet

• Deprecated CSSStyleSheet members are defined.

• The CSSRule.type attribute is deprecated.

• Serialization of <ratio> is added.

• CSSStyleDeclaration.cssText now returns the empty string for computed style.

• Custom properties are included in getComputedStyle.

• MathML IDL is introduced.

• Serialization of CSSKeyframesRule and CSSKeyframeRule is added.

• Serialization of media query is changed.

• A shorthand is not serialized if there are longhands with other property group / mapping logic in between the longhands of that shorthand.

• CSSStyleRule serialization is aware of nesting now.

• Constructable stylesheets is introduced.

11.2. Changes From 5 December 2013

• API for alternative stylesheets is removed: selectedStyleSheetSet, lastStyleSheetSet, preferredStyleSheetSet, styleSheetSets, enableStyleSheetsForSet() on Document.

• The pseudo() method on Element and the PseudoElement interface is removed.

• The cascadedStyle, defaultStyle, rawComputedStyle and usedStyle IDL attributes on Element are removed.

• The cssText IDL attribute’s setter on CSSRule is changed to do nothing.

• IDL attributes of the form webkitFoo (with lowercase w) on CSSStyleDeclaration are added.

• CSSNamespaceRule is changed back to readonly.

• Handling of @charset in insertRule() is removed.

• CSSCharsetRule is removed again.

• Serialization of identifiers and strings is changed.

• Serialization of selectors now supports combinators ">>" and "||" and the "i" flag in attribute selectors.

• Serialization of :lang() is changed.

• Serialization of <color> and <number> is changed.

• setProperty() on CSSStyleDeclaration is changed.

11.3. Changes From 12 July 2011 To 5 December 2013

• Cross-origin stylesheets are not allowed to be read or changed.
• CSSCharsetRule is re-introduced.
• CSSGroupingRule and CSSMarginRule are introduced.
• CSSNamespaceRule is now mutable.
• Parse and serialize a CSS declaration block is now defined.
• Shorthands are now supported in setProperty(), getPropertyValue(), et al.
• setPropertyValue and setPropertyPriority are added to CSSStyleDeclaration.
• The style and media attributes of various interfaces are annotated with the [PutForwards] WebIDL extended attribute.
• The pseudo() method on Element is introduced.
• The PseudoElement interface is introduced.
• The cascadedStyle, rawComputedStyle and usedStyle attributes on Element and PseudoElement are introduced.
• The CSS.escape() static method is introduced.

12. Security Considerations

No new security considerations have been reported on this specification.

13. Privacy Considerations

No new privacy considerations have been reported on this specification.

14. Acknowledgments

The editors would like to thank Alexey Feldgendler, Benjamin Poulain, Björn Höhrmann, Boris Zbasky, Brian Kardell, Chris Dumez, Christian Krebs, Daniel Glazman, David Baron, Domenic Denicola, Dominique Hazael-Massieux, fantasai, Hallvord R. M. Steen, Ian Hickson, John Daggett, Lachlan Hunt, Mike Sherov, Myles C. Maxfield, Morten Stenshorne, Ms2ger, Nazım Can Altınova, Øyvind Stenhaug, Peter Sloetjes, Philip Jägenstedt, Philip Taylor, Richard Gibson, Robert O’Callahan, Simon Sapin, Sjoerd Visscher, Sylvain Galineau, Tarquin Wilton-Jones, Xidorn Quan, and Zack Weinberg for contributing to this specification.

Additional thanks to Ian Hickson for writing the initial version of the alternative style sheets API and canonicalization (now serialization) rules for CSS values.