GeezerVison

1. Introduction

CSS Levels 1 and 2 allowed for the alignment of text via text-align and the alignment of blocks by balancing auto margins. However, except in table cells, vertical alignment was not possible. As CSS adds further capabilities, the ability to align boxes in various dimensions becomes more critical. This module attempts to create a cohesive and common box alignment model to share among all of CSS.

Note: The alignment of text and inline-level content is defined in [CSS-TEXT-3] and [CSS-INLINE-3].

Note: This specification is not intended to change any of the behavior defined in CSS2.1 when the properties defined here are set to their initial values. If implementors or anyone else notices a discrepancy, please report this to the CSSWG as an error.

This section (above) is not normative.

1.1. Module Interactions

This module adds some new alignment capabilities to the block layout model described in [CSS2] chapters 9 and 10, redefines how overconstrained block-level box margins are resolved, and defines the interaction of these new alignment properties with the alignment of table cell content using vertical-align, as defined in [CSS2] chapter 17.

The interaction of these properties with Grid Layout [CSS-GRID-1] and Flexible Box Layout [CSS-FLEXBOX-1] is defined in their respective modules. The property definitions here supersede those in [CSS-FLEXBOX-1] (which have a smaller, earlier subset of permissible values).

No properties in this module apply to the ::first-line or ::first-letter pseudo-elements.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

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.

1.3. Partial Implementations

Since it is expected that support for the features in this module will be deployed in stages corresponding to the various layout models affected, it is hereby clarified that the rules for partial implementations that require treating as invalid any unsupported feature apply to any alignment keyword which is not supported across all layout modules to which it applies for layout models in which the implementation supports the property in general.

For example, if an implementation supports align-self in [CSS-GRID-1] and [CSS-FLEXBOX-1], then it must treat start as invalid unless it is supported in both grid and flex containers. However if that same implementation does not support align-self for block-level elements at all, then a lack of implementation of align-self: start does not trigger this requirement to treat it as invalid.

2. Overview of Alignment Properties

The box alignment properties in CSS are a set of 6 properties that control alignment of boxes within other boxes. They can be described along two axises:

• which dimension they apply to (main/inline vs. cross/block), and
• whether they control the position of the box within its parent, or the box’s content within itself.

Note: This specification uses the terms “justify” and “align” to distinguish between alignment in the main/inline and cross/block dimensions, respectively. The choice is somewhat arbitrary, but having the two terms allows for a consistent naming scheme that works across all of CSS’s layout models (including CSS Flexbox 1 § 2 Flex Layout Box Model and Terminology)

The following table summarizes the box alignment properties and the display types they can apply to.

Common	Axis	Aligns	Applies to
justify-content	main/inline	content within element (effectively adjusts padding)	block containers, flex containers, and grid containers
align-content	cross/block
justify-self	inline	element within parent (effectively adjusts margins)	block-level boxes, absolutely-positioned boxes, and grid items
align-self	cross/block		absolutely-positioned boxes, flex items, and grid items
justify-items	inline	items inside box (controls child items’ justify-self: auto)	block containers and grid containers
align-items	cross/block		flex containers and grid containers

Note: The *-items properties don’t affect the element itself. When set on a container, they specify the interpretation of any *-self: auto used on children of the container element.

3. Alignment Terminology

Since this module defines alignment properties for all layout modes in CSS, some abstract terminology is introduced:

alignment subject

The alignment subject is the thing or things being aligned by the property. For justify-self and align-self, the alignment subject is the margin box of the box the property is set on, and assumes the writing mode of that box. For justify-content and align-content, the alignment subject is defined by the layout mode and refers to some aspect of its contents; it also assumes the writing mode of the box the property is set on.

alignment container

The alignment container is the rectangle that the alignment subject is aligned within. This is defined by the layout mode, but is usually the alignment subject’s containing block, and assumes the writing mode of the box establishing the containing block.

fallback alignment

Some alignments can only be fulfilled in certain situations or are limited in how much space they can consume; for example, space-between can only operate when there is more than one alignment subject, and baseline alignment, once fulfilled, might not be enough to absorb all the excess space. In these cases a fallback alignment takes effect (as defined below) to fully consume the excess space.

4. Alignment Keywords

All of the alignment properties use a common set of keyword values, which are defined in this section. Keywords fall into three categories:

positional alignment

These keywords define alignment as an absolute position within the alignment container.

baseline alignment

These keywords define alignment as a relationship among the baselines of multiple alignment subjects within an alignment context.

distributed alignment

These keywords define alignment as a distribution of space among alignment subjects.

4.1. Positional Alignment: the center, start, end, self-start, self-end, flow-start, flow-end, flex-start, flex-end, left, and right keywords

The positional alignment keywords specify a position for an alignment subject with respect to its alignment container.

Values have the following meanings:

center (self, content)

Centers the alignment subject within its alignment container.

start (self, content)

Aligns the alignment subject to be flush with the alignment container’s start edge in the appropriate axis.

end (self, content)

Aligns the alignment subject to be flush with the alignment container’s end edge in the appropriate axis.

self-start (self)

Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the alignment subject’s start side in the appropriate axis.

self-end (self)

Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the alignment subject’s end side in the appropriate axis.

flow-start (self, content)

Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the start of placement.

In a flex formatting context, this corresponds to the flex container’s main-start or cross-start side, as appropriate. [CSS-FLEXBOX-1]

In grid lanes layout, this corresponds to the side of the grid lanes container from which item placement proceeds in the axis being aligned. See [CSS-GRID-3] for more details.

When used outside of a flex formatting context or grid lanes layout, this value behaves as start, unless otherwise specified by the layout model.

flow-end (self, content)

Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the end of placement.

In a flex formatting context, this corresponds to the flex container’s main-end or cross-end side, as appropriate. [CSS-FLEXBOX-1]

In grid lanes layout, this corresponds to the side of the grid lanes container opposite the side from which item placement proceeds in the axis being aligned. See [CSS-GRID-3] for more details.

When used outside of a flex formatting context or grid lanes layout, this value behaves as end, unless otherwise specified by the layout model.

flex-start (self, content)

A legacy value alias for flow-start.

flex-end (self, content)

A legacy value alias for flow-end.

left (only justify-*)

Aligns the alignment subject to be flush with the alignment container’s line-left or physical left edge, whichever is in the appropriate axis.

If the property’s axis is not parallel with either left↔right axis, this value behaves as start. Currently, the only case where the property’s axis is not parallel with either left↔right axis is in a column flexbox.

right (only justify-*)

Aligns the alignment subject to be flush with the alignment container’s line-right or physical right edge, whichever is in the appropriate axis.

If the property’s axis is not parallel with either left↔right axis, this value behaves as start. Currently, the only case where the property’s axis is not parallel with either left↔right axis is in a column flexbox.

Two grammar terms are used to denote certain subsets of these values:

<self-position>

This set is used by justify-self and align-self to align the box within its alignment container, and also by justify-items and align-items (to specify default values for justify-self and align-self).

<self-position> = center | start | end | self-start | self-end |
        flow-start | flow-end | flex-start | flex-end

<content-position>

This set is used by justify-content and align-content to align the box’s contents within itself.

<content-position> = center | start | end |
        flow-start | flow-end | flex-start | flex-end

Note: left and right are excluded from <self-position> and <content-position>, despite being valid positional alignment values for justify-content/justify-self/justify-items, because they are not allowed in the align-* properties. They are instead explicitly included in the justify-* properties’ grammars.

Make it easier to understand the dual-axis nature of "start" and "end" wrt orthogonal flows.

4.2. Baseline Alignment: the baseline keyword and first/last modifiers

See CSS Writing Modes 3 § 4.1 Introduction to Baselines.

Baseline alignment is a form of positional alignment that aligns multiple alignment subjects within a shared alignment context (such as cells within a row) by matching up their alignment baselines. If the position of the alignment subjects within a baseline-sharing group is not fully constrained by baseline alignment (i.e., they could be shifted within their respective alignment containers while maintaining baseline-alignment), they are fallback-aligned insofar as possible while preserving their baseline alignment.

The baseline alignment keywords are represented with the <baseline-position> grammar term:

<baseline-position> = [ first | last ]? && baseline

The first and last values give a box a baseline alignment preference: either “first” or “last”, respectively, defaulting to “first”.

Values have the following meanings:

baseline

Computes to first baseline, defined below.

first baseline

Specifies participation in first-baseline alignment: aligns the alignment baseline of the box’s first baseline set with the corresponding baseline of its baseline-sharing group. See § 9.3 Aligning Boxes by Baseline for more details.

The fallback alignment for first baseline is safe self-start (for self-alignment) or safe start (for content-distribution).

last baseline

Specifies participation in last-baseline alignment: aligns the alignment baseline of the box’s last baseline set with the corresponding baseline of its baseline-sharing group.̣ See § 9.3 Aligning Boxes by Baseline for more details.

The fallback alignment for last baseline is safe self-end (for self-alignment) or safe end (for content-distribution).

When specified for align-content, these values trigger baseline content-alignment, shifting the content of the box within its content box, and may also affect the sizing of the box itself. See § 5.4 Baseline Content-Alignment.

When specified for align-self/justify-self, these values trigger baseline self-alignment, shifting the entire box within its container, which may affect the sizing of its container. See § 6.4 Baseline Self-Alignment.

When baseline self-alignment is specified on a box, content distribution is performed first, then the resulting box and its contents are self-aligned However, if the box also has baseline content-alignment in the same axis or if its content-distribution property in the same axis is normal, its used content-distribution property in that axis is start or safe end for a baseline alignment preference of its baseline self-alignment of “first” or “last”, respectively.

Add example images here.

If a box does not belong to a shared alignment context, then the fallback alignment is used. For example, align-content: baseline on a block box falls back to start alignment. The fallback alignment is also used to align the baseline-sharing group within its alignment container.

Note: Because they are equivalent, and baseline is shorter, the CSSOM serializes first baseline as baseline. See CSSOM § 6.7.2 Serializing CSS Values.

Note: For the somewhat-related vertical-align property, due to inconsistent design decisions in CSS2.1, baseline is not equivalent to first baseline as an inline-level box’s baseline alignment preference depends on display. (E.g., inline-block uses its last baseline by default, while inline-table uses its first baseline by default.)

4.3. Distributed Alignment: the stretch, space-between, space-around, and space-evenly keywords

The distributed alignment values are used by justify-content and align-content to disperse a container’s extra space among its alignment subjects.

When space cannot be distributed in this way, these values behave as their fallback alignment. Each distribution value has an associated default fallback alignment. (A future level of this module may allow the fallback alignment to be specified explicitly.)

space-between

The alignment subjects are evenly distributed in the alignment container. The first alignment subject is placed flush with the start edge of the alignment container, the last alignment subject is placed flush with the end edge of the alignment container, and the remaining alignment subjects are distributed so that the spacing between any two adjacent alignment subjects is the same.

The default fallback alignment for this value is safe flow-start.

space-around

The alignment subjects are evenly distributed in the alignment container, with a half-size space on either end. The alignment subjects are distributed so that the spacing between any two adjacent alignment subjects is the same, and the spacing before the first and after the last alignment subject is half the size of the other spacing.

The default fallback alignment for this value is safe center.

space-evenly

The alignment subjects are evenly distributed in the alignment container, with a full-size space on either end. The alignment subjects are distributed so that the spacing between any two adjacent alignment subjects, before the first alignment subject, and after the last alignment subject is the same.

The default fallback alignment for this value is safe center.

stretch

If the combined size of the alignment subjects is less than the size of the alignment container, any auto-sized alignment subjects have their size increased equally (not proportionally), while still respecting the constraints imposed by max-height/max-width (or equivalent functionality), so that the combined size exactly fills the alignment container.

The default fallback alignment for this value is flow-start.

Note: This `align-content/stretch` definition applies to the content-distribution properties; the self-alignment properties justify-self/align-self have their own stretch value, which can grow or shrink the alignment subject to ensure it exactly fits the alignment container.

These values are represented with the <content-distribution> grammar term:

<content-distribution> = space-between | space-around | space-evenly | stretch

4.4. Overflow Alignment: the safe and unsafe keywords and scroll safety limits

In some situations, aligning exactly as specified would cause the alignment subject to overflow its alignment container, possibly causing data loss. For example, if the contents of a sidebar are unconditionally centered, items large enough to overflow it might extend beyond the viewport’s start edge, which can’t be scrolled to.

To control this situation, an overflow alignment mode can be explicitly specified. “Unsafe” alignment honors the specified alignment mode in overflow situations, even if it causes data loss, while “safe” alignment changes the alignment mode in overflow situations in an attempt to avoid data loss. The default behavior is to contain the alignment subject within the scrollable area, though at the time of writing this safety feature is not yet implemented.

<overflow-position> = unsafe | safe

safe

If the size of the alignment subject overflows the alignment container, the alignment subject is instead aligned as if the alignment mode were flow-start. However for absolutely positioned boxes, see § 4.4.1.2 Self-Alignment for Absolutely Positioned Boxes.

unsafe

Regardless of the relative sizes of the alignment subject and alignment container, the given alignment value is honored.

default (no value specified)

If the overflow alignment isn’t explicitly specified, the default overflow alignment is a blend of “safe” and “unsafe”. See § 4.4.1 Automatic Overflow Alignment Safety for details.

4.4.1. Automatic Overflow Alignment Safety

If no overflow alignment mode is specified for a property, the default behavior lies somewhere between safe and unsafe, and also varies by layout mode.

4.4.1.1. Content Distribution for Scroll Containers

In a scrollable axis of a scroll container, the default overflow alignment behavior for content distribution is unsafe. Non-normal values of the content-distribution properties instead alter the scrollable overflow area in order to allow access to the overflowing content. See § 5.3 Alignment Overflow and Scroll Containers.

4.4.1.2. Self-Alignment for Absolutely Positioned Boxes

For absolutely positioned alignment subjects whose relevant used self-alignment property is not normal, the default and safe overflow alignment behavior is as follows:

(For absolutely-positioned alignment subjects that fail the above condition, see § 4.4.1.3 All Other Alignment.)

Note: These rules constrain the position of the box to minimize overflow while honoring the specified alignment insofar as possible, and ensuring continuous behavior as the sizes of the boxes change.

Define “scrolls with the page” to be more precise, capturing any anchors that are either contained by or anchored into the scrollable overflow area.

4.4.1.3. All Other Alignment

For all other elements:

1. If the alignment subject overflows its alignment container, align as specified (unsafe).

2. If the alignment subject would overflow the scrollable overflow area of its nearest ancestor scroll container, (thus extending into the “unscrollable” region), then its overflow in that direction is limited by biasing any remaining overflow to the opposite side.

It may not be Web-compatible to implement the “smart” default behavior (though we hope so, and believe it to be likely), so UAs should pass any feedback on this point to the WG. UAs that have not implemented the “smart” default behavior must behave as safe for align-content on block containers and unsafe otherwise.

5. Content Distribution: Aligning a Box’s Contents Within Itself

Content distribution controls alignment of the box’s content within its content box. It is specified by the content-distribution properties align-content and justify-content (and their place-content shorthand).

5.1. The justify-content and align-content Properties

Name:	align-content
Value:	normal | <baseline-position> | <content-distribution> | <overflow-position>? <content-position>
Initial:	normal
Applies to:	block containers, multicol containers, flex containers, and grid containers
Inherited:	no
Percentages:	n/a
Computed value:	specified keyword(s)
Canonical order:	per grammar
Animation type:	discrete

Name:	justify-content
Value:	normal | <content-distribution> | <overflow-position>? [ <content-position> | left | right ]
Initial:	normal
Applies to:	multicol containers, flex containers, and grid containers
Inherited:	no
Percentages:	n/a
Computed value:	specified keyword(s)
Canonical order:	per grammar
Animation type:	discrete

Aligns the contents of the box as a whole (as the alignment subject) within the box itself (as the alignment container): along the inline/row/main axis of the box (for justify-content) or the block/column/cross axis of the box (for align-content). Values other than normal are defined in § 4 Alignment Keywords, above.

For all layout modes, the alignment subject and alignment container both assume the writing mode of the box the *-content property is set on.

5.1.1. Block Containers (Including Table Cells)

Alignment Container	The block container’s content box.
Alignment Subject(s)	The entire contents of the block, as a unit.
align-content Axis	The block axis. If a <content-distribution> is specified its fallback alignment is used instead. If no <overflow-position> is specified, and the block container is not a block-axis scroll container, then alignment is safe.
justify-content Axis	Does not apply to and has no effect on block containers.
normal Behavior	All values other than normal force the block container to establish an independent formatting context. For table cells, the behavior of align-content: normal depends on the computed value of vertical-align: top makes it behave as start and bottom makes it behave as end; otherwise middle makes it behave as center, and all other values make it behave as baseline. [CSS2] normal otherwise behaves as start.

5.1.2. Multicol Containers

Alignment Container	The multi-column container’s content box.
Alignment Subject(s)	The column boxes, with any spacing inserted between column boxes added to the relevant column gaps.
align-content Axis	The block axis, treating the column boxes (and any column-spanning elements), as a singular unit. If a <content-distribution> is specified its fallback alignment is used instead.
justify-content Axis	The inline axis.
normal Behavior	normal behaves as stretch; both are defined as described in the column-sizing rules of [CSS-MULTICOL-1]. In the case of multi-column containers with a non-auto column-width, justify-content values other than normal or stretch cause the columns to take their specified column-width rather than stretching to fill the container. The column boxes are then aligned as specified by justify-content.

5.1.3. Flex Containers

Alignment Container	The flex container’s content box.
Alignment Subject(s)	For justify-content, the flex items in each flex line. For align-content, the flex lines. Note, this only has an effect on multi-line flex containers.
align-content Axis	The cross axis.
justify-content Axis	The justify-content property applies along the main axis, but since stretching in the main axis is controlled by flex, stretch behaves as flow-start.
normal Behavior	normal behaves as stretch.

See [CSS-FLEXBOX-1] for details.

5.1.4. Grid Containers

Alignment Container	The grid container’s content box.
Alignment Subject(s)	The grid tracks in the appropriate axis, with any spacing inserted between tracks added to the relevant gutters, and treating collapsed gutters as a single opportunity for space insertion.
align-content Axis	The block axis, aligning the grid rows.
justify-content Axis	The inline axis, aligning the grid columns.
normal Behavior	normal behaves as stretch.

See [CSS-GRID-1] for details.

5.2. Content-Distribution Shorthand: the place-content property

Name:	place-content
Value:	<'align-content'> <'justify-content'>?
Initial:	normal
Applies to:	see individual properties
Inherited:	no
Percentages:	n/a
Computed value:	see individual properties
Canonical order:	per grammar
Animation type:	discrete

This shorthand property sets both the align-content and justify-content properties in one declaration. The first value is assigned to align-content. The second value is assigned to justify-content; if omitted, it is copied from the first value, unless that value is a <baseline-position> in which case it is defaulted to start.

5.3. Alignment Overflow and Scroll Containers

When the content-distribution properties are set in a scrollable axis of a scroll container with an overflowing alignment subject, they reduce the clipping of the unreachable scrollable overflow region just enough to ensure that alignment subject can be scrolled into its start-aligned position.

Note: The alignment subject is not necessarily identical to the scrollable overflow area: content overflowing the alignment subject—​such as an out-of-flow box—​grows the scrollable overflow area but not the alignment subject. Thus an end-aligned scroll container might not be initially scrolled all the way to the bottom, and positioned content can still be clipped if it is further into the unreachable scrollable overflow region than the in-flow content forming the alignment subject.

Note: The presence of scrollbars can change the size of the scroll container’s content box—​and thus the size of the alignment container and/or alignment subject.

5.4. Baseline Content-Alignment

The content of boxes participating in row-like layout contexts (shared alignment contexts) can be baseline-aligned to each other. Baseline content-alignment effectively increases the padding on the box to align the alignment baseline of its contents with that of other baseline-aligned boxes in its baseline-sharing group.

Note: Implementations that do not fully support baseline content-alignment in applicable formatting contexts must not parse the baseline values as valid. Lack of test coverage in WPT is not an excuse for shipping broken implementations. <—​Currently browsers are parsing it but not doing anything useful with it. :( https://github.com/w3c/csswg-drafts/issues/13421 -->

Baseline content-alignment can only apply if the align-content axis is parallel with the box’s block axis (i.e. it does not apply to “column” flex containers); otherwise the fallback alignment is used.

The set of boxes that participate in baseline content-alignment depends on the layout model:

Table Cells:

A (non-replaced) table cell participates in first/last baseline content-alignment in its startmost/endmost row if its inline axis is parallel to that of the table itself and its computed align-content is first baseline (last baseline).

Note: Baseline content-alignment of cells sharing a column is not supported; however this may be added in a future level if there is sufficient demand and implementer interest.

Flex Items:

A non-replaced flex item participates in first/last baseline content-alignment in its flex line if its computed align-content is first baseline/last baseline and its inline axis is parallel to the main axis.

Grid Items:

A non-replaced grid item participates in first/last baseline content-alignment in its startmost/endmost row or column (whichever is parallel to its inline axis) and if its computed align-content is first baseline/last baseline.

Additionally, in order to participate in baseline content-alignment it must also have a coordinated self-alignment preference, to guarantee that the box lines up the relevant edge with other boxes in its baseline-sharing group. That is, the box’s start (end) margin edge must be intended to align—​and actually align—​to the corresponding edge of its containing block when its baseline alignment preference is “first” (“last”). It otherwise takes its fallback alignment.

When is a self-alignment preference coordinated?

A box’s start margin edge is aligned to the corresponding edge of its containing block and it has a coordinated self-alignment preference for a “first” baseline alignment preference when, in the relevant axis:

• There are no auto margins and the relevant self-alignment property either is or aligns identically to stretch or self-start; or

• There is only an auto end-edge margin, which absorbs any positive free space and disables the effects of any self-alignment property, and its margin box does not overflow its containing block under circumstances that would cause it to effectively end-align instead (such as having a containing block with an opposite writing mode).

A box’s end margin edge is aligned to the corresponding edge of its containing block and it has a coordinated self-alignment preference for a “last” baseline alignment preference when, in the relevant axis:

• There are no auto margins and the relevant self-alignment property either is or aligns identically to self-end and its self-alignment is what would result from an unsafe overflow alignment; or

• There is only an auto start-edge margin, which absorbs any positive free space and disables the effects of any self-alignment property and its margin box does not overflow its containing block under circumstances that would cause it to effectively start-align instead.

See § 9.3 Aligning Boxes by Baseline for additional details. Baseline content-alignment can increase the intrinsic size of the box.

6. Self-Alignment: Aligning the Box Within Its Parent

Self-alignment controls alignment of the box within its containing block. It is specified by the self-alignment properties align-self and justify-self (and their place-self shorthand).

6.1. Inline-Axis (or Main-Axis) Self-Alignment: the justify-self property

Name:	justify-self
Value:	auto | <overflow-position>? [ normal | <self-position> | left | right ] | stretch | <baseline-position>
Initial:	auto
Applies to:	block-level boxes, absolutely-positioned boxes, and grid items
Inherited:	no
Percentages:	n/a
Computed value:	specified keyword(s)
Canonical order:	per grammar
Animation type:	discrete

Justifies the box (as the alignment subject) within its containing block (as the alignment container) along the inline/row/main axis of the alignment container: the box’s outer edges are aligned within its alignment container as described by its alignment value. Values have the following meanings:

auto

Behaves as normal if the box has no parent, or when determining the actual position of an absolutely positioned box. It behaves as the computed justify-items value of the parent box (minus any legacy keywords) otherwise (including when determining the static position of an absolutely positioned box).

normal

Represents the “default” alignment for the layout mode. Its behavior depends on the layout mode, as described below.

<self-position> | left | right

Shifts the position of the box as specified, see § 4 Alignment Keywords.

<overflow-position>

Controls how the box aligns when it overflows, see § 4.4 Overflow Alignment: the safe and unsafe keywords and scroll safety limits.

stretch

When the box’s computed width/height (as appropriate to the axis) is auto and neither of its margins (in the appropriate axis) are auto, sets the box’s used size to the length necessary to make its outer size as close to filling the alignment container as possible while still respecting the constraints imposed by min-height/min-width/max-height/max-width.

Unless otherwise specified, this value falls back to flow-start generally, and to self-start or self-end if the box has also specified first baseline or last baseline baseline content-alignment (respectively) in the same axis.

Note: The stretch keyword can cause elements to shrink, to fit their container.

<baseline-position>

Indicates baseline self-alignment, as defined in § 4.2 Baseline Alignment: the baseline keyword and first/last modifiers, § 6.4 Baseline Self-Alignment, and § 9 Baseline Alignment Details.

Values other than stretch cause a width/height of auto to be treated as fit-content.

Note: auto margins, because they effectively adjust the size of the margin area, take precedence over justify-self.

6.1.1. Block-Level Boxes

justify-self Axis	The block’s containing block’s inline axis, generally. The static-position containing block’s inline axis when determining the static position.
Alignment Container	The box’s containing block, except that for block-level elements that establish a block formatting context and are placed next to a float, the alignment container is reduced by the space taken up by the float, assuming the writing mode of the containing block.
Alignment Subject	The block’s margin box, assuming the writing mode of the block.
normal Behavior	The box lays out according to the default rules for block layout (see CSS2.1§10.3).
Other Details	The automatic size of a block-level box whose justify-self is not normal is equivalent to fit-content (rather than e.g. stretch, as is typical for a non-replaced block-level box). Additionally, in terms of CSS2.1 block-level formatting [CSS2], the rules for “over-constrained” computations in section 10.3.3 are ignored in favor of alignment as specified here and the used value of the margin properties are therefore not adjusted to correct for the over-constraint. This property does not apply to floats. Anonymous block boxes always behave as normal.

6.1.2. Absolutely-Positioned Boxes

This section describes the effect of justify-self on how the margin box of an absolutely-positioned box is positioned with respect to its (absolute-positioning) containing block.

justify-self Axis	The block’s containing block’s inline axis.
Alignment Container	The box’s containing block, as modified by the inset properties (top/right/bottom/left), assuming the writing mode of the containing block. If both inset properties in the relevant axis are auto, then use the box’s static-position rectangle (i.e. set both insets to the box’s static position) and assume the writing mode of the static-position containing block.
Alignment Subject	The box’s margin box, assuming the writing mode of the box.
normal Behavior	Behaves as stretch or start, depending on the type of box. See CSS Positioned Layout 3 § 4 Absolute Positioning Layout Model.
Other Details	In terms of CSS2.1 formatting [CSS2], the rules for “over-constrained” computations in section 10.3.7 are ignored in favor of alignment as specified here, and the used value of the inset properties are not adjusted to correct for the over-constraint. Values other than stretch or normal cause non-replaced absolutely-positioned boxes to use fit-content sizing for calculating auto sizes in the affected axis. Note that stretch does cause replaced absolutely-positioned boxes to fill their containing block just as non-replaced ones do. Note: If only one inset property is auto, the computations in CSS2 section 10.3.7 fully determine its size and position, and justify-self has no effect. (If both are auto, then the box is statically-positioned, see above, and can be aligned within the static-position rectangle.)

6.1.3. Table Cells

This property does not apply to table cells, because their position and size is fully constrained by table layout.

6.1.4. Flex Items

This property does not apply to flex items, because there is more than one item in the main axis. See flex for stretching and justify-content for main-axis alignment. [CSS-FLEXBOX-1]

6.1.5. Grid Items

justify-self Axis	The grid’s inline axis.
Alignment Container	The grid item’s grid area, assuming the writing mode of the grid container.
Alignment Subject	The grid item’s margin box, assuming the writing mode of the grid item.
normal Behavior	Sizes as either stretch (typical non-replaced elements) or start (typical replaced elements); see Grid Item Sizing in [CSS-GRID-1]. The resulting box is then start-aligned.

See [CSS-GRID-1] for details.

6.2. Block-Axis (or Cross-Axis) Self-Alignment: the align-self property

Name:	align-self
Value:	auto | <overflow-position>? [ normal | <self-position> ] | stretch | <baseline-position>
Initial:	auto
Applies to:	flex items, grid items, and absolutely-positioned boxes
Inherited:	no
Percentages:	n/a
Computed value:	specified keyword(s)
Canonical order:	per grammar
Animation type:	discrete

Aligns the box (as the alignment subject) within its containing block (as the alignment container) along the block/column/cross axis of the alignment container: the box’s outer edges are aligned within its alignment container as described by its alignment value. Values have the following meanings:

auto

Behaves as normal if the box has no parent, or when determining the actual position of an absolutely positioned box. It behaves as the computed align-items value of the parent box (minus any legacy keywords) otherwise (including when determining the static position of an absolutely positioned box).

normal

Represents the “default” alignment for the layout mode, as defined below.

<self-position>

Shifts the position of the box as specified, see § 4 Alignment Keywords.

<overflow-position>

Controls how the box aligns when it overflows, see § 4.4 Overflow Alignment: the safe and unsafe keywords and scroll safety limits.

stretch

As defined for justify-self in § 6.1 Inline-Axis (or Main-Axis) Self-Alignment: the justify-self property.

<baseline-position>

Indicates baseline self-alignment, as defined in § 4.2 Baseline Alignment: the baseline keyword and first/last modifiers, § 6.4 Baseline Self-Alignment, and § 9 Baseline Alignment Details.

Note: auto margins, because they effectively adjust the size of the margin area, take precedence over align-self.

6.2.1. Block-Level Boxes

The align-self property does not apply to block-level boxes (including floats), because there is more than one item in the block axis.

6.2.2. Absolutely-Positioned Boxes

This section describes the effect of align-self on how the margin box of an absolutely-positioned box is positioned with respect to its (absolute-positioning) containing block.

align-self Axis	The box’s containing block’s block axis, generally. The static-position containing block’s block axis when determining the static position.
Alignment Container	The box’s containing block, as modified by the inset properties (top/right/bottom/left), assuming the writing mode of the containing block. If both inset properties in the relevant axis are auto, then use the box’s static-position rectangle (i.e. set both insets to the box’s static position) and assume the writing mode of the static-position containing block.
Alignment Subject	The box’s margin box, assuming the writing mode of the box.
normal Behavior	Behaves as stretch or start, depending on the type of box. See CSS Positioned Layout 3 § 4 Absolute Positioning Layout Model.
Other Details	In terms of CSS2.1 formatting [CSS2], the rules for "over-constrained" computations in section 10.6.4 are ignored in favor of alignment as specified here and the used value of the inset properties are not adjusted to correct for the over-constraint. Values other than stretch or normal cause non-replaced absolutely-positioned boxes to use fit-content sizing for calculating auto sizes in the affected axis. Note that stretch does cause replaced absolutely-positioned boxes to fill their containing block just as non-replaced ones do. Note: If only one inset property is auto, the computations in CSS2 section 10.6.4 fully determine its size and position, and align-self has no effect. (If both are auto, then the box is statically-positioned, see above, and can be aligned within the static-position rectangle.)

6.2.3. Table Cells

This property does not apply to table cells, because their position and size is fully constrained by table layout.

6.2.4. Flex Items

align-self Axis	The flex container’s cross axis.
Alignment Container	The flex line the flex item is in, assuming the writing mode of the flex container.
Alignment Subject	The flex item’s margin box, assuming the writing mode of the flex item.
normal Behavior	Behaves as stretch.

See [CSS-FLEXBOX-1] for details.

6.2.5. Grid Items

align-self Axis	The grid’s block axis.
Alignment Container	The grid item’s grid area, assuming the writing mode of the grid container.
Alignment Subject	The grid item’s margin box, assuming the writing mode of the grid item.
normal Behavior	Sizes as either stretch (typical non-replaced elements) or start (typical replaced elements); see Grid Item Sizing in [CSS-GRID-1]. The resulting box is then start-aligned.

See [CSS-GRID-1] for details.

6.3. Self-Alignment Shorthand: the place-self property

Name:	place-self
Value:	<'align-self'> <'justify-self'>?
Initial:	auto
Applies to:	see individual properties
Inherited:	no
Percentages:	n/a
Computed value:	see individual properties
Canonical order:	per grammar
Animation type:	discrete

This shorthand property sets both the align-self and justify-self properties in a single declaration. The first value is assigned to align-self. The second value is assigned to justify-self; if omitted, it is copied from the first value.

6.4. Baseline Self-Alignment

Boxes participating in row-like layout contexts (shared alignment contexts) can be baseline-aligned to each other. Baseline self-alignment effectively increases the margins on the box to align its alignment baseline with that of other baseline-aligned boxes in its baseline-sharing group.

The set of boxes that participate in baseline self-alignment depends on the layout model:

Flex Items:

A flex item participates in first/last baseline self-alignment in its flex line if its computed align-self is first baseline/last baseline. See [CSS-FLEXBOX-1] for details.

Grid Items:

A grid item participates in first/last baseline self-alignment in its startmost/endmost row or column if its align-self or justify-self property (respectively) computes to first baseline/last baseline.

See § 9.3 Aligning Boxes by Baseline for exact details. Baseline self-alignment can increase the intrinsic size contribution of the box.

6.5. Effects on Sizing of Absolutely Positioned Boxes with Static-Position Insets

For absolutely-positioned boxes whose inline-axis offsets are both auto, the available space for calculating the inline size is also affected by alignment.

Note: In [CSS2], the available space is keyed off of the direction property of the static-position containing block. (See CSS2§10.3.7 and CSS2§10.3.8.) Fundamentally these rules set one of the auto insets (by default, the start-edge inset) to the corresponding edge of the static-position rectangle and the other to the corresponding edge of the containing block (i.e. set the inset to zero). Just as the self-alignment properties replace the containing block’s direction lookup for placement, they also replace this lookup for sizing, as specified here.

Thus, when interpreting the rules in CSS2§10.3.7 and CSS2§10.3.8, wherever the direction property of the static-position containing block is referenced, instead reference the value of the align-self or justify-self property (whichever is defined to apply to the relevant axis), treating left-equivalent alignment as defined for ltr and right-equivalent alignment as defined for rtl. Treat normal as start and any distributed alignment value as its fallback alignment.

In the case of center alignment, the available space for the box is double the distance between the center of the static-position rectangle and the closest edge of the containing block in the relevant axis.

Note: The align-self/justify-self properties can also modify additional aspects of sizing: for example, justify-self: stretch will replace “shrink-to-fit” (fit-content) sizing into the available space with stretch-fit sizing (consuming all of the available space). This is an independent effect from the available space adjustment here.

Note: This only affect how available space is calculated for sizing the absolutely-positioned box; its alignment is as specified in previous sections.

7. Default Alignment

The align-items and justify-items properties (and their place-items shorthand) set the default align-self and justify-self behavior of the element’s child boxes.

7.1. Inline-Axis (or Main-Axis) Default Alignment: the justify-items property

Name:	justify-items
Value:	normal | stretch | <baseline-position> | <overflow-position>? [ <self-position> | left | right ] | legacy | legacy && [ left | right | center ]
Initial:	legacy
Applies to:	all elements
Inherited:	no
Percentages:	n/a
Computed value:	specified keyword(s), except for legacy (see prose)
Canonical order:	per grammar
Animation type:	discrete

This property specifies the default justify-self for all of the child boxes (including anonymous boxes) participating in this box’s formatting context. Values have the following meanings:

legacy

This keyword causes the value to effectively inherit into descendants.

If the legacy keyword appears on its own (without an accompanying left, right, or center keyword): if the inherited value of justify-items includes the legacy keyword, this value computes to the inherited value; otherwise it computes to normal.

When justify-self:auto references the value of justify-items, only the alignment keyword, not the legacy keyword, is referenced by it. It exists to implement the legacy alignment behavior of HTML’s <center> element and align attribute.

Other values have no special handling and are merely referenced by justify-self.

7.2. Block-Axis (or Cross-Axis) Default Alignment: the align-items property

Name:	align-items
Value:	normal | stretch | <baseline-position> | <overflow-position>? <self-position>
Initial:	normal
Applies to:	all elements
Inherited:	no
Percentages:	n/a
Computed value:	specified keyword(s)
Canonical order:	per grammar
Animation type:	discrete

This property specifies the default align-self for all of the child boxes (including anonymous boxes) participating in this box’s formatting context.

Values have no special handling and are merely referenced by align-self.

7.3. Default Alignment Shorthand: the place-items property

Name:	place-items
Value:	<'align-items'> <'justify-items'>?
Initial:	see individual properties
Applies to:	all elements
Inherited:	no
Percentages:	n/a
Computed value:	see individual properties
Canonical order:	per grammar
Animation type:	discrete

This shorthand property sets both the align-items and justify-items properties in a single declaration. The first value is assigned to align-items. The second value is assigned to justify-items; if omitted, it is copied from the first value.

8. Gaps Between Boxes

This section has been moved to CSS Gaps 1 § 2 Gaps Between Boxes.

9. Baseline Alignment Details

Boxes in a baseline-sharing group are aligned to each other using their alignment baselines. For example, in horizontal writing modes, specifying align-content: baseline on table cells in the same row will align the baselines of their first formatted lines. This section defines exactly how baseline alignment is performed in consideration of the myriad baselines and writing modes that exist in internationalized modern CSS.

A baseline set is a set of baselines (alphabetic, central, etc.) associated with a common baseline table. Typically, a typesetting tradition will use only one of these, but different writing systems use different baselines, and mixing writing systems can result in using more than one within a single line. Refer to CSS Writing Modes 3 § 4.1 Introduction to Baselines for more information on baselines and writing modes.

9.1. Determining the Baselines of a Box

Each box, for a given axis, has potentially a first baseline set (and last baseline set) that nominally corresponds to the baseline set of the first/last line of text within the box. The alignment baseline, which is the baseline used to align the box in its alignment context, is one of the baselines in its baseline set, usually the dominant baseline associated with the shared alignment context. (See the dominant-baseline and alignment-baseline properties in [CSS-INLINE-3].)

The first and last baseline sets of a box are determined differently based on the layout model, as follows:

line box

The first/last baseline set of a line box is generated from the dominant baseline and the font settings of its root inline box.

block containers

The first/last baseline set of a block container is taken from the first/last in-flow line box in the block container or the first/last in-flow block-level child in the block container that contributes a set of first/last baselines, whichever comes first/last. If there is no such line box or child, then the block container has no baseline set.

However, for legacy reasons if its baseline-source is auto (the initial value) a block-level or inline-level block container that is a block-axis scroll container always has a last baseline set, whose baselines all correspond to its block-end margin edge.

multi-column containers

The first baseline set of a multi-column container is the first baseline set of the column or multi-column spanner with the highest (block-start–most) baseline corresponding to the multi-column container’s alignment baseline. If there is no such line box or child, then the multi-column container has no first baseline set.

The last baseline set is analogous, but uses the last baseline set and lowest (block-end–most) baseline.

tables

The first/last baseline set of a table box is the first/last baseline set of its first/last row (after any reordering of table row group box).

When finding the first/last baseline set of an inline-block, any baselines contributed by table boxes must be skipped. (This quirk is a legacy behavior from [CSS2].)

table rows

The first baseline set of a table row box is taken from the first available of:

1. If any cells in the row participate in first baseline alignment along the table’s inline axis, generate a baseline set from their shared alignment baseline and the row’s first available font, after alignment has been performed.

2. Otherwise, if any cells in the row participate in last baseline alignment along the table’s inline axis, generate from that alignment baseline.

3. Otherwise, if the row is not empty, synthesize from the lowest and highest content edges of all the cells in the row. See CSS 2 § 17.5.3 Table height algorithms.

4. Otherwise, use the block-start content edge of the table row box itself as the alignment baseline.

For this purpose, any table cell that spans multiple rows is ignored if it’s span does not start in this row; except that for step 2, it’s ignored if its span does not end in this row.

Last baselines are analogous (with “first”/“last” and “start”/“end” inverted).

Spanning cells participate only in the first/last row that they span for the purpose of first baseline/last baseline.

flex containers

See Flex Baselines in [CSS-FLEXBOX-1].

grid containers

See Grid Baselines in [CSS-GRID-1].

To generate baselines for a box from a single baseline, use the baseline table from the font settings and first available font of that box, and align that baseline set to the given single baseline.

If a box that participates in baseline alignment has no baseline set, then its alignment baseline is synthesized according to the rules of the formatting context in which it participates. To synthesize baselines from a rectangle (or two parallel lines), synthesize the alphabetic baseline from the line-under line, and the central baseline by averaging the positions of the two edges or lines. See CSS Inline Layout 3 § A Synthesizing Alignment Metrics for rules on synthesizing additional baselines.

Note: The edges used to synthesize baselines from a box depend on their formatting context: inline-level boxes synthesize from their margin edges [CSS-INLINE-3], table cells synthesize from their content edges [CSS2], and grid and flex items synthesize from their border edges [CSS-GRID-1] [CSS-FLEXBOX-1].

In general, the writing mode of the box, shape, or other object being aligned is used to determine the line-under and line-over edges for synthesis. However, when that writing mode’s block flow direction is parallel to the axis of the alignment context, an axis-compatible writing mode must be assumed:

• If the box establishing the alignment context has a block flow direction that is orthogonal to the axis of the alignment context, use its writing mode.

• Otherwise:

  • If the box’s own writing mode is vertical, assume horizontal-tb.

  • If the box’s own writing mode is horizontal, assume vertical-lr if direction is ltr and vertical-rl if direction is rtl.

For the purposes of finding the baselines of a box, it and all its in-flow descendants that are block-axis scroll containers must be considered as if scrolled to their initial scroll position. Additionally, if the position of a scroll container’s first/last baseline is outside its border edge, that baseline’s position is clamped to the border edge.

9.2. Baseline Alignment Grouping

A baseline-sharing group is composed of boxes that participate in baseline alignment together. This is possible only if they both:

• Share an alignment context along an axis perpendicular to the axis they’re being baseline-aligned in. (For example, grid items with align-self: baseline are baseline-aligning along the grid’s block axis, and therefore participate with other items in their row.)
• Have compatible baseline alignment preferences (i.e., the baselines that want to align are on the same side of the alignment context).

Note: Boxes participating in baseline content-alignment and boxes participating in baseline self-alignment can be part of the same baseline-sharing group, and can thus align to each other. The difference between the two methods is in where extra space is inserted to perform the alignment (inside or outside the box’s own border).

Boxes share an alignment context, along a particular axis, and established by a particular box, when they are:

• table cells in the same row, along the table’s row (inline) axis, established by the row box
• grid items in the same row, along the grid’s row (inline) axis, established by the grid container
• grid items in the same column, along the grid’s column (block) axis, established by the grid container
• flex items in the same flex line, along the flex container’s main axis, established by the flex container

Note: Conceptually, the inline-level boxes in a line box also share a self-alignment context and participate in a baseline-sharing group; however they only baseline-align in response to the vertical-align property, not any of the properties defined in this module. See [CSS-INLINE-3].

If a box spans multiple shared alignment contexts, then it participates in first/last baseline alignment within its start-most/end-most shared alignment context along that axis. For example, a table cell spanning three rows participates in first-baseline alignment with the table cells in the first row that it spans, or alternatively in last-baseline alignment with the table cells in the last row that it spans.

The baseline alignment preferences of two boxes in a baseline-sharing group are compatible if they have:

• the same block flow direction and same baseline alignment preference

• opposite block flow direction and opposite baseline alignment preference

9.3. Aligning Boxes by Baseline

Given a set of alignment subjects and their baselines that all belong to a single baseline-sharing group, the alignment subjects are baseline-aligned as follows:

1. Generate the baseline-sharing group’s baseline table from the first available font of the group’s alignment context and overlay also the mirror of this baseline table by aligning their central baselines. These are the baseline “grids” to which the alignment subjects will align.

2. Align each alignment subject by its specified alignment baseline to the group’s baseline table or its mirror, whichever matches the alignment subject’s line orientation. Unless otherwise specified (e.g. via the alignment-baseline property), the alignment baseline is the dominant baseline of the alignment context.

3. Position the aligned baseline-sharing group within the alignment container according to its fallback alignment. The fallback alignment of a baseline-sharing group is the fallback alignment of its items as resolved to physical directions.

4. For first/last baseline content-alignment, then add the minimum necessary extra space between the alignment container’s start/end content edge and the alignment subject’s edge to align the start/end margin edges of all the alignment containers in the alignment context while maintaining baseline alignment within the baseline-sharing group.

Appendix A: Static Position Terminology

When both inset properties in a given axis are auto on an absolutely positioned box, CSS2 uses its static position to resolve its size and position. See CSS2.1§10.3.7. The box alignment properties modify these calculations, just as they do the sizing and positioning calculations in other layout modes. These modifications refer to a static-position rectangle, whose edges represent the static position of the box from each side of its containing block.

The static-position rectangle and the static positions to which it corresponds are defined by the layout model of its “hypothetical box”:

Block Layout

The static positions of a block-level box are defined in [CSS2] Chapter 10. The static-position rectangle is a zero-thickness rectangle spanning between the inline-axis sides of the box’s static-position containing block (see CSS2§10.3.7); and positioned at its block-start static position (see CSS2§10.6.4).

Inline Layout

The static positions of an inline-level box are defined in [CSS2] Chapter 10. The static-position rectangle is a zero-thickness rectangle spanning between the line-over/line-under sides of the line box that would have contained its “hypothetical box” (see CSS2§10.3.7); and positioned at its inline-start static position.

Flex Layout

The static-position rectangle of the child of a flex container corresponds to the content edges of the flex container. See CSS Flexbox 1 § 4.1 Absolutely-Positioned Flex Children.

Grid Layout

By default, the static-position rectangle of the child of a grid container corresponds to the content edges of the grid container. However, if that grid container also establishes the box’s actual containing block, then the grid area specified by the grid-placement properties establishes its static-position rectangle instead. See the static position of a grid container child in [CSS-GRID-1].

10. Changes

Changes since the 30 January 2026 Working Draft include:

• Added flow-start and flow-end as value aliases of flex-start and flex-end, with canonical flow-start/flow-end serialization. (Issue 14052)

• Move gap properties to [css-gaps-1]. (Issue 13312)

• Use safe to constrain absolutely positioned boxes to their original containing block. (Issue 14008)

• Improve algorithm for finding baselines of tables and grid containers. (Issue 7655, Issue 7641)

• Clarify special handling of scroll containers to address each axis independently now that a container can be scrollable in only one axis. (Issue 12289)

• Add a reminder that fundamentally unsupported values must not be parsed as valid. (13421)

See also previous changes.

11. Privacy Considerations

As a simple layout spec, this introduces no new privacy considerations.

12. Security Considerations

As a simple layout spec, this introduces no new security considerations.

Acknowledgments

Special thanks goes to David Baron, Javier Fernandez, Markus Mielke, Alex Mogilevsky, and the participants in the CSSWG’s March 2008 F2F alignment discussions for their contributions to the alignment model described herein, and to Melanie Richards for her illustrations of the various alignment keywords.