CSS Box Alignment Module Level 3

1. Introduction

This section is not normative.

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.

1.1. Module interactions

This module adds some new alignment capabilities to the block layout model described in [CSS2] chapters 9 and 10 and defines the interaction of these 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. Values

This specification follows the CSS property definition conventions from [CSS2]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. 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 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, 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.

flex-start (self, content)

Only used in flex layout. [CSS-FLEXBOX-1] Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the flex container’s main-start or cross-start side, as appropriate.

When used outside of a flex formatting context, this value behaves as start. That is, on boxes that are not flex items (or pretending to be flex items, such as when determining the static position of an absolutely-positioned box that is a child of a flex container), this value behaves as start when used in the self-alignment properties, and on boxes that are not flex containers, this value behaves as start when used in the content-distribution properties.

flex-end (self, content)

Only used in flex layout. Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the flex container’s main-end or cross-end side, as appropriate.

When used outside of a flex formatting context, this value behaves as end. That is, on boxes that are not flex items (or pretending to be flex items, such as when determining the static position of an absolutely-positioned box that is a child of a flex container), this value behaves as end when used in the self-alignment properties, and on boxes that are not flex containers, this value behaves as end when used in the content-distribution properties.

left (self, content)

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.

right (self, content)

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.

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 |
        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 | flex-start | flex-end;

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

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 end.

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.

If both baseline content-alignment and baseline self-alignment are specified in the same axis on the same box, only baseline self-alignment is honored in that axis; the content-alignment in that axis must be treated as start.

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 1 §5.7.2 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 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 flex-start. (For layout modes other than flex layout, flex-start is identical to 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 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 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 flex-start. (For layout modes other than flex layout, flex-start is identical to start.)

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

When the alignment subject is larger than the alignment container, it will overflow. Some alignment modes, if honored in this situation, may cause data loss: for example, if the contents of a sidebar are centered, when they overflow they may send part of their boxes past 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 start.

unsafe

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

(no value specified)

If the overflow alignment isn’t explicitly specified, the default overflow alignment is a blend of "safe" and "unsafe" in that an overflowing alignment subject is allowed to overflow its alignment container, but if this would cause it to also overflow the scrollable overflow region 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 unsafe.

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

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)

5.1.2. Multicol Containers

5.1.3. Flex Containers

See [CSS-FLEXBOX-1] for details.

5.1.4. Grid Containers

See [CSS-GRID-1] for details.

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

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. Overflow and Scroll Positions

The content-distribution properties also affect the initial scroll position, setting it to display the appropriate portion of the scrollable area. In other words, the scrollable overflow region is aligned relative to the viewport as specified by the content-distribution properties. Additionally, so that the content remains in the scrollable overflow region in order to be scrollable, the scroll and auto values of overflow trigger safe content alignment always.

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.

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

Table Cells:

A 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 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 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, if self-alignment applies to the box in an axis, in order to participate in baseline content-alignment in that axis it must also have a coordinated self-alignment preference; that is:

• If its baseline alignment preference is “first” (“last”), the relevant computed self-alignment property is either 'align-self/stretch or self-start (self-end''). For this purpose, the start, end, flex-start, and flex-end values are treated as either self-start or self-end, whichever they end up equivalent to.

• Its used self-alignment is not affected by its overflow alignment. (That is, the used self-alignment is what would result from an unsafe overflow alignment.)

It otherwise takes its fallback alignment.

See §9.3 Aligning Boxes by Baseline for exact 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/Main-Axis Alignment: the justify-self property

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.

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 flex-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.

<overflow-position>? <self-position>

Defined in §4 Alignment Keywords, above.

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

6.1.2. Absolutely-Positioned Boxes

This section describes the effect of justify-self on how the margin box of an absolutely-positioned box which is not statically-positioned is positioned with respect to its (absolute-positioning) containing block.

6.1.3. Static Position of Absolutely-Positioned Boxes

This section describes the effect of justify-self on how the margin box of an absolutely-positioned box which is statically-positioned is positioned with respect to its static-position rectangle.

6.1.4. Table Cells

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

6.1.5. 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]

6.1.6. Grid Items

See [CSS-GRID-1] for details.

6.2. Block/Cross-Axis Alignment: the align-self property

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.

stretch

As defined for justify-self in §6.1 Inline/Main-Axis 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.

<overflow-position>? <self-position>

Defined in §4 Alignment Keywords, above.

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 which is not statically-positioned is positioned with respect to its (absolute-positioning) containing block.

6.2.3. Static Position of Absolutely-Positioned Boxes

This section describes the effect of align-self on how the margin box of an absolutely-positioned box which is statically-positioned is positioned with respect to its static-position rectangle.

6.2.4. Table Cells

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

6.2.5. Flex Items

See [CSS-FLEXBOX-1] for details.

6.2.6. Grid Items

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

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.

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/Main-Axis Alignment: the justify-items property

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/Cross-Axis Alignment: the align-items property

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. Self-Alignment Shorthand: the place-items property

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

While margin and padding can be used to specify visual spacing around individual boxes, it’s sometimes more convenient to globally specify spacing between adjacent boxes within a given layout context, particularly when the spacing is different between boxes as opposed to between the first/last box and the container’s edge.

The gap property, and its row-gap and column-gap sub-properties, provide this functionality for multi-column, flex, and grid layout.

8.1. Row and Column Gutters: the row-gap and column-gap properties

These properties specify fixed-length gutters between items in the container, adding space between them—in a manner to the space-between keyword of the content-distribution properties, but of a fixed size instead of as a fraction of remaining space. The column-gap property specifies spacing between “columns”, separating boxes in the container’s inline axis similar to inline-axis margin; while row-gap indicates spacing between “rows”, separating boxes in the container’s block axis.

Values have the following meanings:

<length-percentage>

Specifies a gap between “rows” or “columns”, as defined by the layout modes to which it applies; see subsections below for details.

Negative values are invalid. Percentages resolve to zero when specified against a content-based size (such as the logical width of a float or the auto logical height of a block-level grid container).

normal

The normal represents a used value of 1em on multi-column elements, and a used value of 0px in all other contexts.

Gutters effect a minimum spacing between items: additional spacing may be added by justify-content/align-content. Such additional space effectively increases the size of these gutters.

The exact handling of these properties varies by layout container:

multi-column elements

column-gap specifes the gutter between adjacent column boxes. See [CSS-MULTICOL-1] for details on how this affects the layout of multicol elements. row-gap does not currently apply.

flex containers

When applied to the main axis (e.g. column-gap in a row flex container), indicates minimum spacing between items (as if an additional fixed-size margin were inserted between adjacent flex items in a single line).

When applied to the cross axis (e.g. row-gap in a row flex container), indicates minimum spacing between adjacent flex lines.

grid containers

The row-gap and column-gap properties, when specified on a grid container, define the gutters between grid rows and grid columns, respectively. See CSS Grid Layout 1 §10.1 Gutters: the row-gap, column-gap, and gap properties for precise details.

Note: Table boxes do not use the gap properties to specify separation between their cells. Instead, they use the border-spacing property, which has slightly different functionality: it inherits, and it also specifies the additional spacing between the outermost cells and the border of the table (similar to space-evenly rather than space-between).

8.2. Gap Shorthand: the gap property

This property is a shorthand that sets row-gap and column-gap in one declaration. If <‘column-gap’> is omitted, it’s set to the same value as <‘row-gap’>.

Note: The gap property is only one component of the visible “gutter” or “alley” created between boxes. Margins, padding, or the use of distributed alignment may increase the visible separation between boxes beyond what is specified in gap.

8.3. Legacy Gap Properties: the grid-row-gap, grid-column-gap, and grid-gap properties

The Grid Layout module was originally written with its own set of gutter properties, before all such properties were unified into the existing row-gap/column-gap naming. For compatibility with legacy content, those legacy property names must be supported as aliases:

• grid-row-gap must be treated as a shorthand for the row-gap property

• grid-column-gap must be treated as a shorthand for the column-gap property

• grid-gap must be treated as a shorthand for the gap property

In all three cases, the legacy properties must take the same grammar as the property they are aliasing, and just "forward" the value to their aliased property.

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 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 is one of these, 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:

block containers

The first (last) baseline set of a block container is generated from the dominant first (last) baseline of the first (last) in-flow line box in the block container, or is taken from 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.

multi-column containers

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

Multi-column containers have no last baseline set.

tables

The first (last) baseline set of a table box is the first (last) baseline set of its first (last) row.

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

If any cells in the row participate in first baseline (last baseline) alignment along the row axis, the first (last) baseline set of the row is generated from their shared alignment baseline and the row’s first available font, after alignment has been performed. Otherwise, the first (last) baseline set of the row is synthesized from the lowest and highest content edges of the cells in the row. [CSS2]

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.

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 § Synthesizing Baselines 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, assume either horizontal-tb or vertical-lr writing-mode, whichever is orthogonal to the box’s own writing-mode.

For the purposes of finding the baselines of a box, it and all its in-flow descendants with a scrolling mechanism (see the overflow property) must be considered as if scrolled to their origin. Furthermore, if, in the case of a scroll container box, the resulting position of a first (last) baseline is past a box’s end (start) border edge, its position is clamped to that 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

• 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).

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 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:

First, 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.

Next, 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.

Position the aligned baseline-sharing group within the alignment container according to the fallback alignment associated with the specified baseline alignment preference. 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.

10. Changes

Changes since the 6 September 2017 Working Draft include:

• Added more illustrations! Many thanks to Melanie Richards for working through a difficult diagramming problem.
• Fixed errors in property definition table of gap shorthand.
• Defining percentage gaps to resolve to zero when the containing block’s size depends on the gaps. (Issue 509, Issue 2297)
• Fixed errors in handling left and right in the grammars.
• Fixed discrepency in space-align fallback alignment. (Issue 2316)
• Tightened up handling of baseline alignment in cases where the box edges don’t align. (Issue 1556, Issue 1611)
• Miscellaneous clarifications.

Changes since the 20 July 2017 Working Draft include:

• Added the row-gap and column-gap properties and gap shorthand, applying them to both grid layout and flex layout to replace the grid-specific grid-row-gap/grid-column-gap/grid-gap properties.

Changes since the 15 May 2017 Working Draft include:

• Dropped the left and right keywords from align-self and align-content. They may be re-introduced in a later level if there is demand. (Issue 1403)
• Assigned end fallback alignment to stretch cases when combined with last baseline content-alignment.
• Disabled baseline alignment across cells in a table column, due to implementation complexity.
• A large number of minor corrections and clarifications reported by David Baron.

Changes since the 7 April 2017 Working Draft include:

• Adding a section to clarify handling of partial implementations. (Issue 1167)
• Dropped <baseline-position> value of justify-content since it can’t do anything anyway. (Issue 1184)
• Deferred ability to specify fallback alignments to Level 4. (Issue 1002)
• Fixed position of <overflow-position> wrt <content-position>. (Issue 1001)
• Fixed rules for handling baseline-aligned boxes being aligned along their block axis. (Issue 1038)
• Renamed justify-items: auto to justify-items: legacy. (Issue 1318)
• Clamped baselines of scrollable boxes to the border edge, rather than margin edge. (Issue 766)

11. Privacy and Security Considerations

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