Efficiently rendering a website relies on the user agent being able to detect what parts of the page are being displayed,
which parts might affect the currently-displayed section,
and what can be ignored.
There are various heuristics that can be used to guess when a given sub-tree is independent of the rest of the page in some manner,
but they’re fragile,
so innocuous changes to a page may inadvertently make it fail such heuristic tests,
causing rendering to fall into a slow code path.
There are also many things that would be good to isolate which are difficult or impossible to detect in a heuristic manner.
To alleviate these problems
and allow strong, predictable isolation of a subtree from the rest of the page,
this specification defines a contain property.
To allow even further optimization of off-screen contents,
this spec also defines a content-visibility property,
enabling the user agent to skip an element’s layout and painting entirely when not needed.
1.1.
Module Interactions
This document defines new features not present in earlier specifications.
In addition, it aims to replace and supersede [CSS-CONTAIN-1]
once stable.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the CSS-wide keywords as their property value.
For readability they have not been repeated explicitly.
User agents are expected to support this property on all media, including non-visual ones.
The contain property allows an author to indicate that an element and its contents are,
as much as possible,
independent of the rest of the document tree.
This allows user agents to utilize much stronger optimizations when rendering a page using contain properly,
and allows authors to be confident that their page won’t accidentally fall into a slow code path
due to an innocuous change.
none
This value indicates that the property has no effect.
The element renders as normal,
with no containment effects applied.
strict
This value computes to size layout paint style,
and thus turns on all forms of containment for the element.
Tests
Note:contain: content is reasonably "safe" to apply widely;
its effects are fairly minor in practice,
and most content won’t run afoul of its restrictions.
However, because it doesn’t apply size containment,
the element can still respond to the size of its contents,
which can cause layout-invalidation to percolate further up the tree than desired.
Use contain: strict when possible,
to gain as much containment as you can.
size
The value turns on size containment for the element.
This ensures that the containment box can be laid out
without needing to examine its descendants.
Tests
This value turns on layout containment for the element.
This ensures that the containment box is totally opaque for layout purposes;
nothing outside can affect its internal layout,
and vice versa.
Tests
This value turns on style containment for the element.
This ensures that,
for properties which can have effects on more than just an element and its descendants,
those effects don’t escape the element.
Tests
This value turns on paint containment for the element.
This ensures that the descendants of the containment box don’t display outside its bounds,
so if an element is off-screen or otherwise not visible,
its descendants are also guaranteed to be not visible.
Tests
contain is useful when used widely on a page,
particularly when a page contains a lot of "widgets" which are all independent.
For example, assume a micropost social network had markup something like this:
<body><aside>...</aside><section><h2>Messages</h2><article>
Lol, check out this dog: images.example.com/jsK3jkl
</article><article>
I had a ham sandwich today. #goodtimes
</article><article>
I have political opinions that you need to hear!
</article>
…
</section></body>
There are probably a lot of messages displayed on the site,
but each is independent and won’t affect anything else on the site.
As such, each can be marked with contain: content to communicate this to the user agent,
so it can optimize the page and skip a lot of computation for messages that are off-screen.
If the size of each message is known ahead of time,
contain: strict can be applied to communicate further restrictions.
Note: Several properties beyond contain can turn on various containments for an element.
These do not affect the value of contain;
an element can have contain: none
but still have layout containment turned on by content-visibility,
for example.
3.
Types of Containment
There are several varieties of containment that an element can be subject to,
restricting the effects that its descendants can have on the rest of the page in various ways.
Containment enables much more powerful optimizations by user agents,
and helps authors compose their page out of functional units,
as it limits how widely a given change can affect a document.
Specification authors introducing new properties or mechanisms
need to consider whether and how the various types of containment
affect what they are introducing,
and include in their specification any effect not described here.
3.1.
Size Containment
Giving an element size containment
makes its principal box a size containment box
and has the following effects:
Note: This affects explicit invocations of the min-content or max-content keywords,
as well as any calculation that depends on these measurement,
such as sizing grid tracks into which a size contained item is placed,
or if fit-content sizing the containment box’s parent.
All CSS properties of the size containment box are taken into account
as they would be when performing layout normally.
Other specifications may make specific exemptions.
Note: Even when the element’s sizing properties specify an intrinsic size,
this does not necessarily make the element zero-sized:
properties set on the element itself
continue to be taken into account,
which can cause it to be larger.
Note: Internal table boxes,
which do not include table captions,
are excluded,
because the table layout algorithm
does not allow boxes to become smaller than their inflow content.
Sizing a table cell as if it was empty and then laying out its content inside without changing the size
is effectively an undefined operation.
Manually setting the width or height properties to 0
cannot make it smaller than its content.
This concern does not apply to table captions,
which are perfectly capable of having a fixed size
that is independent of their content.
By itself, size containment does not offer much optimization opportunity.
Its primary benefit on its own is that tools which want to lay out the containment box’s contents
based on the containment box’s size
(such as a JS library implementing the "container query" concept)
can do so without fear of "infinite loops",
where having a child’s size respond to the size of the containment box
causes the containment box’s size to change as well,
possibly triggering further changes in how the child sizes itself
and possibly thus more changes to the containment box’s size,
ad infinitum.
When paired with layout containment, though,
possible optimizations that can be enabled include (but are not limited to):
When the style or contents of a descendant of the containment box is changed,
calculating what part of the DOM tree is "dirtied" and might need to be re-laid out
can stop at the containment box.
When laying out the page,
if the containment box is off-screen or obscured,
the layout of its contents (i.e. "laying out in-place") can be delayed or done at a lower priority.
3.2.
Inline-Size Containment
Giving an element inline-size containment
applies size containment to the inline-axis sizing of its principal box.
This means the inline-axisintrinsic sizes of the principal box
are determined as if the element had no content.
However, content continues to impact the box’s block-axisintrinsic sizes as usual,
and the box is allowed to fragment normally in the block axis.
Note: In some cases,
a box’s block-axisintrinsic sizes
can impact layout in the parent formatting context
in ways that affect the box’s inline size
(e.g. by triggering scrollbars on an ancestor element),
creating a dependency of the box’s inline size on its own content.
If this changed inline size results in a different block size,
that new block size can loop into further impacting the parent formatting context,
but not in a way that reverts it to the previously-problematic layout.
For example, if scrollbars were introduced,
they are then not removed,
even if the consequent block size is small enough to not need them;
or if a box’s logical height collides with a lower-placed float and is cleared down
to where it also has more available inline space
and thus becomes short enough to not have collided,
it is then not moved back up to its previous problematic size and position.
In general, the relationship between an element’s inline size
and it’s block size
is unpredictable and non-monotonic,
with the block size capable of shifting up and down arbitrarily
as the inline size is changed.
Infinite cycles are prevented
by ensuring that layout does not revert to a previous (known-problematic) state,
even if a naive analysis of the constraints would allow for such;
in other words, layout always “moves forward”.
We believe that current CSS layout specifications incorporate such rules,
but to the extent that they don’t,
please inform the CSSWG
so that these errors can be corrected.
Consider this example,
where float placement creates a dependency of block sizes on inline sizes:
The block layout algorithm will first place the floating boxes,
with the first two sitting in the left and right corners of the container,
and the third, being too wide to fit between, being pushed below them.
The following article will then be laid out.
Because it is display: flow-root,
it cannot intersect any floats,
and thus must take them into account
when figuring out how to size and position itself.
The layout engine first attempts to place the article
flush with the top of the container,
resulting a 100px width,
plenty wide enough to accommodate its min-content size.
However, due to the aspect-ratio of its child,
this would cause the article to be 100px tall as well,
which would intersect the third float 80px below,
so this layout opportunity is discarded.
It then attempts to position the article
flush with the top of the third float,
in the narrow 40px-wide space to its right.
However, since the article’s min-width makes it too large
to fit in the 40px-wide space beside the third float,
it shifts below that one as well,
forming a 200px square below all the floated boxes.
Article
If the min-width is removed from the article,
or if inline-size containment is added to
either the article or header
(causing min-width: min-content to resolve to zero),
then the article will fit as a 40px square
next to the final floated div
(possibly with some of its content overflowing).
At this point, the width and height of the article
(40px each)
would fit back in the first considered space,
flush with the top of the container.
However, the box is not returned to the previous position,
because the layout engine knows already
that this position would result in an invalid layout.
If at least one fragmentation container of a fragmentation context has layout containment,
or if at least one fragmentation container of a fragmentation context is a descendant of layout containment boxand at least one subsequent fragmentation container of the same fragmentation context
is not a descendant of that same element with layout containment,
then the first layout containment box
which is either a fragmentation container itself
or is an ancestor of a fragmentation container
must “trap” the remainder of the fragmented flow:
fragmentation must not continue past the layout containment boundary,
and the last fragmentation container
within the first layout containment boundary
is treated as if it is the last fragmentation container
in its fragmentation context.
If subsequent fragmentation containers in the fragmentation context
are only generated when more content remains in the fragmented flow,
then they are not generated.
If they would exist regardless,
they remain part of the fragmentation context,
but do not receive any content from the fragmented flow.
Note: At the time of writing, no stable specification is affected by this point.
Only specifications that would enable some (but not all) fragmentation containers of a fragmentation context
to be layout-contained (or descendants of a layout contained element)
are concerned.
This is not the case of [CSS-PAGE-3] nor of [CSS-MULTICOL-1].
This requirement is nonetheless included because
several mechanisms that would make this a possibility have been considered
(e.g.: [CSS-REGIONS-1], ::nth-fragment(), a hypothetical selector for individual columns of a multicol…),
and the guarantees that layout containment is intended to offer would not be realized
if such mechanisms did not abide by this rule.
[CSS-REGIONS-1] has details over how layout containment affects
regions.
In this [CSS-REGIONS-1] example,
content can flow from #a to #b,
from #b to #c.
However as #c is the last fragment container in the first layout containment box
it traps all the remaining content,
and nothing gets flowed into #d, #e, or #f.
If the computed value of the overflow property is
either visible or clip or a combination thereof,
any overflow must be treated as ink overflow.
For the purpose of the vertical-align property,
or any other property whose effects
need to relate the position of the layout containment box’s baseline
to something other than its descendants,
the containment box is treated as having no baseline.
Possible optimizations that can be enabled by layout containment include (but are not limited to):
When laying out the page,
the contents of separate containment boxes
can be laid out in parallel,
as they’re guaranteed not to affect each other.
When laying out the page,
if the containment box is off-screen or obscured
and the layout of the visible parts of the screen do not depend on the size of the containment box
(for example, if the containment box is near the end of a block container,
and you’re viewing the beginning of the block container),
the layout of the containment box' contents can be delayed or done at a lower priority.
(When paired with size containment,
this optimization can be applied more liberally.)
3.4.
Style Containment
Giving an element style containment
has the following effects:
Note: This implies that the depth of quote nesting in the subtree
is unchanged and starts at the value that its context normally implies,
but that changes to the depth of quote nesting by these values inside the subtree
do not affect the depth of quote nesting outside the subtree.
A scoped property has its effects scoped to a particular element or subtree.
If scoped to an element,
it must act as if the scoping element was the root of the document
for the purpose of evaluating the property’s effects:
any uses of the property outside the scoping element must have no effect on the uses of the property on or in the scoping element,
and vice versa.
Note: “Scoping to an element” is currently unused.
It is defined as an extension point for future specifications to use.
If scoped to a sub-tree, it’s the same,
except the scoping element itself is counted as "outside" the tree,
like the rest of the document,
and the effects of the property on that element are unaffected by scoping.
When considering the effects of the scoped property on elements inside the subtree,
the element at the base of the subtree is treated as if it was the root of the document.
As counter-increment is scoped to an element’s subtree,
the first use of it within the subtree
creates a fresh counter,
regardless of whether the counter had been used outside the scoping element.
Any increments made within the subtree have no effect on counters of the same name outside the scoping element.
However, the counter() and counters() value of the content property is not itself scoped,
and can refer to counters established outside of the subtree.
body {counter-reset: foo;}
div {counter-increment: foo;/* and to help observe nesting properly */
margin: .2em;padding:.2em;background: #0001;}
div::before {content:counters(foo,".");}
Will render like:
1
2
2.1
2.2
3
4
5
Here, the style-contained element still interacts with the foo counter normally,
incrementing it to 2.
Its ::beforepseudo-element,
which is effectively a child and thus within the containment boundary,
can still see that counter normally,
and thus will render the string "2".
Later siblings of the style-contained element don’t see anything that goes on within the boundary,
so they increment the foo counter to 3 and beyond.
Within the containment, however,
the descendant divs aren’t allowed to modify the value of the outside counter,
so instead they’ll create a fresh (nested) foo counter
when they attempt to increment it.
3.4.1.
Possible Style-Containment Optimizations
This section is non-normative.
Possible optimizations that can be enabled by style containment include (but are not limited to):
Whenever a property is changed on a descendant of an element with style containment,
calculating what part of the DOM tree is "dirtied" and might need to have its style recalculated
can stop at the element with style containment.
3.5.
Paint Containment
Giving an element paint containment
makes its principal box a paint containment box
and has the following effects:
The contents of the element
including any ink or scrollable overflow
must be clipped to the overflow clip edge of the paint containment box,
taking corner clipping into account.
This does not include the creation of any mechanism
to access or indicate the presence of the clipped content;
nor does it inhibit the creation of any such mechanism through other properties,
such as overflow, resize, or text-overflow.
Note: The behavior is described in this paragraph is equivalent to changing
overflow-x: visible into overflow-x: clip
and
overflow-y: visible into overflow-y: clip
at used value time,
while leaving other values of overflow-x and overflow-y unchanged.
Possible optimizations that can be enabled by paint containment include (but are not limited to):
If the containment box is off-screen or obscured,
the UA can usually skip trying to paint its contents,
as they’re guaranteed to be off-screen/obscured as well.
Note: Some paint effects such as the blur() filter from [FILTER-EFFECTS-1]
have non local effects.
The user agent needs to keep track of these,
as it may need to repaint parts of an element with such a filter
when its descendents change,
even if they have paint containment
and could otherwise be skipped.
Unless the clipped content is made accessible via a separate mechanism
such as the overflow or resize properties,
the UA can reserve "canvas" space for the box exactly the box’s size.
(In similar, scrollable, situations, like overflow: hidden,
it’s possible to scroll to the currently-clipped content,
so UAs often predictively overpaint somewhat
so there’s something to see as soon as the scroll happens,
rather than a frame later.)
Because they are guaranteed to be stacking contexts,
scrolling elements can be painted into a single GPU layer.
4. Suppressing An Element’s Contents Entirely: the content-visibility property
Tests
Tests related to content-visibility which haven't been triaged to any particular point in the spec
these should be more accurately dispatched paragraph per paragraph to the relevant points in the specification
The content-visibility property
controls whether or not an element renders its contents at all,
along with forcing a strong set of containments,
allowing user agents to potentially omit large swathes of layout and rendering work
until it becomes needed.
It has the following values:
visible
No effect.
The element’s contents are laid out and rendered as normal.