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This module contains the features of CSS relating to the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout. (The alignment of text and inline-level content is defined in [CSS3TEXT] and [[CSS3LINEBOX]].) CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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This document was produced by the CSS Working Group (part of the Style Activity).
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This is the first public working draft.
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
CSS3 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.
The alignment of text and inline-level content is defined in [CSS3TEXT] and [[CSS3LINEBOX]].
Inspiration for this document:
This module adds some new alignment capabilities to the block layout
model described in [CSS21] chapters 9 and 10 and defines
the interaction of these properties with the alignment of table cell
content using ‘vertical-align
’, as defined
in [CSS21] chapter
17. The interaction of these properties with Grid Layout [CSS3-GRID-LAYOUT] and
Flexible Box Layout [CSS3-FLEXBOX] is defined in
their respective modules.
No properties in this module apply to the ::first-line
or
::first-letter
pseudo-elements.
This specification follows the CSS property
definition conventions from [CSS21]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS
modules may expand the definitions of these value types: for example
[[CSS3VALUES]], when combined with this module, adds the ‘initial
’ keyword as a possible property value.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.
The alignment properties in CSS can be described along two axes:
This proposal uses the terms ‘justify
’ and ‘align
’ to distinguish between alignment in the
inline and stacking 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.
The following table summarizes the proposed alignment properties and the display types they can apply to.
Common | Axis | Aligns | Applies to |
---|---|---|---|
‘justify-self ’
| inline | block-level elements and grid items | |
‘align-self ’
| stacking | flex items and grid items | |
‘justify-content ’
| inline | block containers and flex containers | |
‘align-content ’
| stacking | block containers and flex containers | |
‘justify-items ’
| inline | align/justify-self:
auto ’)
| grid containers |
‘align-items ’
| stacking | flex containers and grid containers |
The exact behavior of these properties on layout models other than Flexbox is still to be determined. This is a First Public Working Draft and is NOT STABLE.
The ‘-items
’ values don't
affect the element itself. When set on a flex container or grid container,
they specify the interpretation of any ‘align/justify-self: auto
’ used on the items in the
container element.
True alignment vs. safe alignment. Maybe make safe by
default, true if ‘true
’ is
specified? Or safe/true depending on layout model (e.g. safe for blocks,
true for flexbox)? Current draft introduces a ‘true
’ keyword, but other options should be
considered.
The ‘justify-self
’ and ‘align-self
’
properties control alignment of the box within its containing block.
justify-self
’ propertyName: | justify-self |
---|---|
Value: | auto | [ start | end | center ] && true? |
Initial: | auto |
Applies to: | block-level elements and grid items |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Animatable: | no |
Canonical order: | per grammar |
Justifies the box within its parent along the inline/row axis: the box's outer edges are aligned within its containing block as described below.
auto
’
justify-content
’ property of the containing
block. For grid items, indicates that the element should stretch to fill
its slot.
start
’
center
’
end
’
true
’
In terms of CSS2.1 block-level formatting, the rules for
"over-constrained" computations in section 10.3.3 are used only when the
element's margin box overflows the containing block and the ‘true
’ keyword is not specified.
Otherwise those rules are ignored in favor of alignment as specified
above, and the used value of the end margin is not adjusted to correct for
the over-constraint.
The 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.
For block-level elements that establish a block formatting context and
are placed next to a float, alignment is with respect to the available
space (subtracting out from the containing block measure the space taken
up by the float), not with respect to the containing block. (Note: This is
the legacy behavior of HTML align
.)
This property will replace ‘grid-column-align
’.
align-self
’ propertyName: | align-self |
---|---|
Value: | auto | [ baseline | head | foot | center | stretch ] && true? |
Initial: | auto |
Applies to: | flex items and grid items |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Animatable: | no |
Canonical order: | per grammar |
Aligns the box within its parent along the block/column/cross axis: the box's outer edges are aligned within its containing block as described below.
auto
’
align-items
’
property of the flex container. For grid items, computes to ‘stretch
’.
baseline
’
baseline
’-aligned siblings
in the same row/line.
head
’
foot
’
center
’
stretch
’
true
’
For flex items, if the "head" and "foot" are not in the alignment dimension, the "start" and "end" sides are used instead.
The ‘justify-content
’ and ‘align-content
’
properties control alignment of the box's content within the box.
justify-content
’ propertyName: | justify-content |
---|---|
Value: | auto | [ start | end | left | right | flex-start | flex-end | center | space-between | space-around ] && true? |
Initial: | auto |
Applies to: | block containers and flex containers |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Animatable: | no |
Canonical order: | per grammar |
Gives the default alignment of the box's content along the inline/row
axis. For block containers, this simply changes the meaning of ‘justify-self: auto
’ on its children. For flex
containers, this aligns the contents of each flex line within its flex
line box. See [[!CSS3FLEXBOX]] for exact details.
auto
’
display: block
’ and
‘justify-self: auto
’, computes to the value
inherited from the parent. Otherwise computes to ‘start
’.
start
’
end
’
flex-start
’
start
’.
end
’
end
’.
left
’
auto
’ if the
line-left direction is in the wrong dimension.
right
’
auto
’ if the
line-right direction is in the wrong dimension.
center
’
space-between
’
space-around
’
true
’
space-between
’ or
‘space-around
’ is
applied) is wider than the containing block.
For flex containers, if the "start" and "end" are not in the alignment dimension, the "head" and "foot" sides are used instead, respectively.
This would, in conjunction with ‘text-align
’, implement the HTML align
attribute and <center>
. The weird behavior of ‘auto
’ is for that and because
‘display: block
’ elements are essentially
transparent, layout-wise, so passing through the alignment unchanged is
useful and probably intuitive to authors (though it's really weird for
implementers).
align-content
’ propertyName: | align-content |
---|---|
Value: | auto | [ head | foot | flex-start | flex-end | baseline | center | space-between | space-around ] && true? |
Initial: | auto |
Applies to: | block containers and flex containers |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Animatable: | no |
Canonical order: | per grammar |
Aligns the box's content within the box along the block/column/cross axis. Values have the following meanings:
auto
’
head
’, except that it does not cause the block to
establish a block formatting context. For table cells, defers alignment
to ‘vertical-align
’ as specified in [CSS21] chapter 17.
For flex containers, it causes any positive free space to be distributed
equally across the lines. [CSS3-FLEXBOX]
head
’
center
’
foot
’
flex-start
’
head
’.
end
’
foot
’.
baseline
’
baseline
’-aligned cells/items in the same row.
This is equivalent to the behavior of ‘vertical-align:
baseline
’ on table cells; see [CSS21] chapter 17 for details. If
size constraints prevent the baselines from fully aligning, alignment is
as close as possible. If the content's position is not fully determined
by baseline alignment, the content is head-aligned insofar as possible
while preserving the baseline alignment. (Content that has no first-line
baseline is thus also head-aligned.)
space-between
’
space-between
’ to
lines in a block container.
space-around
’
space-between
’ to lines in a block container.
true
’
All values other than ‘auto
’ cause a block box to establish a block
formatting context such that its margins do not collapse with its children
and floats do not intrude into its content box.
For flex containers, if the "head" and "foot" are not in the alignment dimension, the "start" and "end" sides are used instead, respectively.
The ‘align-items
’ and ‘justify-items
’
properties, when set on a flex or grid container, set the default ‘align-self
’ and
‘justify-self
’ behavior of the items contained
by the element.
justify-items
’ propertyThis could be applied to grid elements and set the default
alignment of the grid items. It mainly exists because Flexbox wants ‘align-items
’; see
below.
Name: | justify-items |
---|---|
Value: | auto | [ start | center | end ] && true? |
Initial: | auto |
Applies to: | grid containers |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Animatable: | no |
Canonical order: | per grammar |
Sets the default ‘justify-self
’ of the grid container's items.
align-items
’ propertyName: | align-items |
---|---|
Value: | auto | [ head | foot | flex-start | flex-end | center | baseline | stretch ] && true? |
Initial: | auto |
Applies to: | flexbox and grid containers |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Animatable: | no |
Canonical order: | per grammar |
Sets the default ‘align-self
’ of the flex container's children
or the grid container's items. The ‘auto
’ value computes to ‘stretch
’ on flex and grid containers.
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:
This is an example of an informative example.
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.
Conformance to CSS Box Alignment Module Level 3 is defined for three conformance classes:
A style sheet is conformant to CSS Box Alignment Module Level 3 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 CSS Box Alignment Module Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Box Alignment Module Level 3 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 CSS Box Alignment Module Level 3 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.
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 component 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.
To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
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.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
[Change or remove the following CR exit criteria if the spec is not a module, but, e.g., a Note or a profile. This text was decided on 2008-06-04.]
For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
The specification will remain Candidate Recommendation for at least six months.
Special thanks goes to Markus Mielke, Alex Mogilevsky, and the participants in the CSSWG's March 2008 F2F alignment discussions.
auto
’, 3.1., 3.2., 4.1., 4.2.
baseline
’, 3.2., 4.2.
center
’, 3.1., 3.2., 4.1., 4.2.
end
’, 3.1., 4.1., 4.1., 4.2.
flex-start
’, 4.1., 4.2.
foot
’, 3.2., 4.2.
head
’, 3.2., 4.2.
left
’, 4.1.
right
’, 4.1.
space-around
’, 4.1., 4.2.
space-between
’, 4.1., 4.2.
start
’, 3.1., 4.1.
stretch
’, 3.2.
true
’, 3.1., 3.2., 4.1., 4.2.
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
align-content | auto | [ head | foot | flex-start | flex-end | baseline | center | space-between | space-around ] && true? | auto | block containers and flex containers | no | N/A | visual |
align-items | auto | [ head | foot | flex-start | flex-end | center | baseline | stretch ] && true? | auto | flexbox and grid containers | no | N/A | visual |
align-self | auto | [ baseline | head | foot | center | stretch ] && true? | auto | flex items and grid items | no | N/A | visual |
justify-content | auto | [ start | end | left | right | flex-start | flex-end | center | space-between | space-around ] && true? | auto | block containers and flex containers | no | N/A | visual |
justify-items | auto | [ start | center | end ] && true? | auto | grid containers | no | N/A | visual |
justify-self | auto | [ start | end | center ] && true? | auto | block-level elements and grid items | no | N/A | visual |