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 interactionsThis 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.
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 ImplementationsSince 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 PropertiesThe 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:
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.
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 TerminologySince this module defines alignment properties for all layout modes in CSS, some abstract terminology is introduced:
All of the alignment properties use a common set of keyword values, which are defined in this section. Keywords fall into three categories:
The positional alignment keywords specify a position for an alignment subject with respect to its alignment container.
Values have the following meanings:
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.
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.
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.
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> = center | start | end | self-start | self-end |
flex-start | flex-end;
<content-position> = center | start | 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.
The start and end keywords are flow-relative: they use the writing mode to determine which side to align to. start vs end in the inline axis (typically† justify-*)The left and right keywords are absolute (not flow-relative).
left vs rightThe start and end keywords have meaning in both the inline and block axes: start always orients to the start of the text (top left for left-to-right/top-to-bottom languages like English) while end always orients to the end of the text.
start vs end in the block axis (typically† align-*)The start and end keywords use the writing mode of the alignment container, to help keep things consistent. But if alignment using the alignment subject’s writing mode is needed, the self-start and self-end keywords can be used.
start vs self-start on the individual items The behavior of the alignment keywords is analogous in vertical writing modes: start and end are relative to the start/end of the text in the relevant axis. The left and right keywords are interpreted as line-left and line-right, relative to the “left” and “right” sides of LTR/RTL text. start vs end in the inline axis (typically† justify-*) left vs right start vs end in the block axis (typically† align-*) start vs self-start on the individual itemsFor most layout models (block, table, grid, etc), the
justify-*properties always align things in the
inline axis, while the
align-*properties always align things in the
block axis.
Flexbox, on the other hand, has justify-* align things in the main axis and align-* align things in the cross axis. This depends on the value of flex-direction: when flex-direction is row or row-reverse, it matches the other layout modes (inline axis with justify-*, block axis with align-*); when flex-direction is column or column-reverse, it has the opposite correspondence.
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 modifiersSee CSS Writing Modes 3 §4.1 Introduction to Baselines.
Baseline alignment is a form of positional alignment that aligns multiple alignment subjects within a (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:
The fallback alignment for first baseline is start.
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.
If a box does not belong to a , 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 keywordsThe distributed alignment values are used by justify-content and align-content to disperse a container’s extra space among its alignment subjects.
The distributed alignment valuesWhen 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.)
The default fallback alignment for this value is flex-start. (For layout modes other than flex layout, flex-start is identical to start.)
The default fallback alignment for this value is center.
The default fallback alignment for this value is center.
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 | stretch4.4. 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
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.
The figure below illustrates the difference in "safe" versus "unsafe" centering, using a column flexbox as an example:
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Authoritarianism
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About
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The items in the figure on the left are set to align-self: safe center;, while those in the figure on the right are set to align-self: unsafe center;. If this column flex container was placed against the left edge of the page, the "safe" behavior would be more desirable, as the long item would be fully readable. In other circumstances, the "unsafe" centering behavior might be better, as it correctly centers all the items. 5. Content Distribution: Aligning a Box’s Contents Within ItselfContent-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 PropertiesAligns 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 ContainersSee [CSS-FLEXBOX-1] for details.
5.1.4. Grid ContainersSee [CSS-GRID-1] for details.
5.2. Content-Distribution Shorthand: the place-content propertyThis 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 PositionsWhen the content-distribution properties are set on a scroll container with overflowing content, rather than shifting the layout positions of its content, they instead change its initial scroll position so that the initially-visible content of the scroll container satisfies the expected alignment of the alignment subject and alignment container.
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.
For example, if a scrollable flex container is set to
justify-content: flex-end(or
justify-content: flex-startwith
flex-flow: row-reverse), it will be initially displayed scrolled all the way to the main-end edge of its content, and its content will appear to overflow its main-start edge.
Because the scroll container’s end-edge padding is not always included in the scrollable overflow region (see CSS Overflow 3 §2.2 Scrollable Overflow), performing such alignment might require that the alignment subject be positioned in a way that would normally not be possible just by scrolling. To resolve this problem (and be consistent across alignment values), all non-normal values of align-content/justify-content cause the scrollable overflow region to be extended in the relevant axis however much would be required to make it possible to scroll to a position satisfying end alignment in that axis. (In general, this means that the scroll container’s padding is added to the bounding rectangle of its in-flow children on both sides of the scrollable overflow region. See [css-overflow-3] for details.)
Coordinate wording with css-overflow once it’s less of a mess.
None of this changes how scroll coordinates are assigned: the origin is still where it would be in a start-aligned container, it just might be initially positioned outside the scrollport.
Note: The alignment subject is not necessarily identical to the scrollable overflow region: content overflowing the alignment subject, such as an absolutely-positioned or out-of-flow element, grows the scrollable overflow region but not the alignment subject, thus an end-aligned scroll container might not be initially scrolled all the way to the bottom.
5.4. Baseline Content-AlignmentThe content of boxes participating in row-like layout 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:
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.
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:
The relevant self-alignment property behaves as either stretch or self-start (self-end) given a baseline alignment preference of ”first” (“last”). 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 ParentSelf-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 propertyJustifies 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:
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).
Represents the “default” alignment for the layout mode. Its behavior depends on the layout mode, as described below.
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.
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.
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 BoxesThe effect of these rules is that an auto-sized block-level table, for example, can be aligned while still having side margins. If the table’s max-content size is narrower than its containing block, then it is shrink-wrapped to that size and aligned as specified. If the table’s max-content size is wider, then it fills its containing block, and the margins provide appropriate spacing from the containing block edges.
6.1.2. Absolutely-Positioned BoxesThis 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). Alignment Subject The box’s margin box, assuming the writing mode of the box. normal BehaviorFor replaced absolutely-positioned boxes, the normal keyword behaves as start.
For all other absolutely-positioned boxes, normal behaves as stretch. Additionally, if neither inset in this axis is auto, but width (height, for vertical writing modes) is auto, auto margins are treated as zero.
(In non-normal cases, auto margins are prioritized over the self-alignment properties, as usual.)
Note: This somewhat bizarre behavior is required for compat with the behavior specified in CSS 2.1.
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 CellsThis property does not apply to table cells, because their position and size is fully constrained by table layout.
6.1.4. Flex ItemsThis 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 ItemsSee [CSS-GRID-1] for details.
6.2. Block/Cross-Axis Alignment: the align-self propertyAligns 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:
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).
Represents the “default” alignment for the layout mode, as defined below.
As defined for justify-self in §6.1 Inline/Main-Axis Alignment: the justify-self property.
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.
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 BoxesThe 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 BoxesThis 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.
align-self Axis The box’s containing block’s block 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). Alignment Subject The box’s margin box, assuming the writing mode of the box. normal BehaviorFor replaced absolutely-positioned boxes, the normal keyword behaves as start.
For all other absolutely-positioned boxes, normal behaves as stretch. Additionally, if neither inset in this axis is auto, but width (height, for vertical writing modes) is auto, auto margins are treated as zero.
(In non-normal cases, auto margins are prioritized over the self-alignment properties, as usual.)
Note: This somewhat bizarre behavior is required for compat with the behavior specified in CSS 2.1.
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 CellsThis property does not apply to table cells, because their position and size is fully constrained by table layout.
6.2.4. Flex ItemsSee [CSS-FLEXBOX-1] for details.
6.2.5. Grid Items 6.3. Self-Alignment Shorthand: the place-self propertyThis 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-AlignmentBoxes participating in row-like layout 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:
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 InsetsFor absolutely-positioned boxes whose inline-axis offsets are both auto, the available space for calculating the inline size is also affected by alignment.
In [CSS2], this is keyed off of the 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 for placement, they also replace this lookup for sizing. 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 normal as start and any distributed alignment value as its fallback alignment.
Instead of always sizing within the available space between the inline-start static position and the inline-end containing block edge, an absolutely-positioned element with auto insets will be sized with reference to the static position rectangle’s edge(s) most appropriate to its specified self-alignment.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.
7. Default AlignmentThe 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 propertyThis 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:
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 propertyThis 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 propertyThis 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 BoxesWhile 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 propertiesThese 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:
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. As in the min size properties and margins/paddings [CSS-SIZING-3], percentages resolve against zero for determining intrinsic size contributions, but resolve against the box’s content box when laying out the box’s contents.
The normal represents a used value of 1em on multi-column containers, 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:
When applied to the cross axis (e.g. row-gap in a row flex container), indicates minimum spacing between adjacent flex lines.
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 propertyThis 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 propertiesThe 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 DetailsBoxes 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 BoxEach 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 . (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:
However, for legacy reasons, if an inline-block is a scroll container or contains no in-flow line boxes, its first and last baseline sets are synthesized from its margin box.
Multi-column containers have no last baseline set.
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].)
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 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 , an axis-compatible writing mode must be assumed:
If the box establishing the 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 initial scroll position. 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 GroupingA baseline-sharing group is composed of boxes that participate in baseline alignment together. This is possible only if they
Boxes share an , along a particular axis, and established by a particular box, when they are:
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 , 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
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 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 .
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 while maintaining baseline alignment within the baseline-sharing group.
10. Appendix A: Static Position TerminologyWhen 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 CSS 2.1 §10.3.7 Absolutely positioned, non-replaced elements. 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”:
Changes since the 23 April 2018 Working Draft include:
Changes since the 6 September 2017 Working Draft include:
Changes since the 20 July 2017 Working Draft include:
Changes since the 15 May 2017 Working Draft include:
Changes since the 7 April 2017 Working Draft include:
As a simple layout spec, this introduces no new privacy or security considerations.
AcknowledgmentsSpecial 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.
Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:
Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.
A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.
A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.
The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.
Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.
To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.
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