[css3-flexbox] …message topic…
"
Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
The specification describes a CSS box model optimized for user interface design. In the flexbox layout model, the children of a flexbox can be laid out in any direction, and can "flex" their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent. Both horizontal and vertical alignment of the children can be easily manipulated. Nesting of these boxes (horizontal inside vertical, or vertical inside horizontal) can be used to build layouts in two dimensions.
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.
The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-flexbox” in the subject, preferably like this: “[css3-flexbox] …summary of comment…”
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.
display
’ property
flex
’ property
This section is not normative.
CSS 2.1 defined four layout modes — algorithms which determine the size and position of boxes based on their relationships with their sibling and ancestor boxes: block layout, designed for laying out documents; inline layout, designed for laying out text; table layout, designed for laying out information in a tabular format; and positioned layout, designed for very explicit positioning without much regard for other elements in the document. This module introduces a new layout mode, flexbox layout, which is designed for laying out more complex applications and webpages.
Flexbox layout is superficially similar to block layout. It lacks many
of the more complex text or document-formatting properties that can be
used in block layout, such as ‘float
’ and ‘columns
’,
but in return it gains more simple and powerful tools for aligning its
contents in ways that webapps and complex web pages often need.
The contents of a flexbox can be laid out in any direction (left, right, down, or even up!), can have their order swapped around dynamically (i.e., display order is independent of source order), and can "flex" their sizes and positions to respond to the available space. If a flexbox is multi-line, the flexbox items flow in two dimensions, wrapping into separate lines in a fashion similar to how text is wrapped into multiple lines.
For example, the following HTML snippet uses flexbox to create a toolbar with icons. The flexbox is horizontal, and the children's widths don't fill the flexbox's width, so the additional space is distributed around and between the children. As the flexbox grows (perhaps because the user is viewing the page on a wider screen), the children spread out evenly and automatically:
<ul> <li><button><img src='new.svg' alt="New"></button></li> <li><button><img src='upload.svg' alt="Upload"></button></li> <li><button><img src='save.svg' alt="Save"></button></li> <li><button><img src='trash.svg' alt="trash"></button></li> </ul> <style> ul { display: flexbox; flex-pack: distribute; } /* Irrelevant styling for this example removed. */ </style>
Example rendering of the above code snippet, at two different flexbox widths.
This module extends the definition of the ‘display
’ property.
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 [CSS3COLOR], when combined with this module, expands the definition of the <color> value type as used in this specification.
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.
An element with ‘display:flexbox
’ or
‘display:inline-flexbox
’ is a flexbox. Children of a flexbox are called flexbox
items and are laid out using the flexbox box model.
Unlike block layout, which is normally biased towards laying things out vertically, and inline layout, which is normally biased toward laying things out horizontally, the flexbox layout algorithm is agnostic as to the direction the flexbox happens to be laid out in. To make it easier to talk about flexbox layout in a general way, we will define several direction-agnostic terms here to make the rest of the spec easier to read and understand.
An illustration of the various directions and sizing
terms used in this specification, respectively for ‘row
’ and ‘column
’
flexboxes.
The main axis of a flexbox is the axis on which
flexbox items are laid out along. The
flexbox items are ordered such that
they start on the main-start side of the flexbox,
and go toward the main-end side. A flexbox item's width or height, whichever
is in the main axis, is the item's main size. The flexbox
item's main size property is
either the ‘width
’ or ‘height
’ property, whichever is in the main axis.
The axis perpendicular to the main axis
is called the cross axis, and similarly has cross-start and cross-end
directions and sides defined. The width or height of a flexbox item, whichever is in the cross axis, is the item's cross size, and similarly the cross size property is whichever of
‘width
’ or ‘height
’ that is in the cross axis.
The contents of a flexbox can be easily and powerfully manipulated with
a handful of properties. Most significantly, flexbox items can "flex" their main size by using the ‘flex
’ property. This
"flexing" allows the items to get bigger or smaller based on the available
space in the page. If there is leftover space in the flexbox after all of
the flexbox items have finished
flexing, the items can be aligned, centered, or distributed with the
‘flex-pack
’ property. Flexbox items can also be completely
rearranged within the flexbox with the ‘flex-order
’ property.
In the cross axis, flexbox items can either "flex" to fill
the available space or be aligned within the space with the ‘flex-align
’
property. If a flexbox is multi-line,
new lines are added in the cross-end
direction, and can similarly be aligned, centered, or distributed within
the flexbox with the ‘flex-line-pack
’ property.
display
’ propertyName: | display |
---|---|
New value: | flexbox | inline-flexbox |
You can declare that an element is a flexbox, and thus should use
flexbox layout for its contents, by setting the ‘display
’ property on the element to the value
‘flexbox
’
or ‘inline-flexbox
’.
The ‘flexbox
’
value makes the flexbox a block-level element. The ‘inline-flexbox
’ value makes the flexbox an
inline-level element. When it is necessary to distinguish them, this
specification will refer to the former kind of flexbox as a block flexbox
and the latter type as an inline flexbox.
Flexboxes use a new layout algorithm, and so some properties that were designed with the assumption of block layout don't make sense in a flexbox context. In particular:
column-*
’ properties in
the Multicol module have no effect on a flexbox.
float
’ and ‘clear
’ have no effect on a flexbox item.
Using ‘float
’ on an element still
causes that element's ‘display
’
property to compute to ‘block
’, as
normal, because that occurs before flexbox items are determined (the
algorithm for wrapping children of a flexbox into flexbox items needs the computed value of
‘display
’).
vertical-align
’ has no effect
on the alignment of a flexbox item in the flexbox
If an element's specified value for ‘display
’ is ‘inline-flexbox
’ and the element is floated or
absolutely positioned, the computed value of ‘display
’ must be ‘flexbox
’.
A flexbox establishes a new flexbox formatting context for its contents. This is similar to a block formatting context root: floats do not intrude into the flexbox, and the flexbox's margins do not collapse with the margins of its contents. Additionally, each of the flexbox items establish a new formatting context for its contents.
Flexbox layout algorithm operates on flexbox items, which are boxes that satisfy at least one of the following criteria:
Some values of ‘display
’ trigger
"fixup" to ensure a sensible tree structure. For example, a lone
‘table-cell
’ box is fixed up by
generating ‘table
’, ‘table-row-group
’, and ‘table-row
’ boxes around it. This fixup must occur
before a flexbox's contents are checked to see if it's necessary
to generate anonymous flexbox items.
In the future, other kinds of fixup such as ‘display:run-in
’ or ‘display:ruby
’ should also run before flexbox fixup.
Examples of flexbox items:
<div style="display:flexbox"> <!-- flexbox item: block-level child --> <div id="item1">block</div> <!-- not a flexbox item, because it's out-of-flow --> <div id="not-an-item1.5" style="position: absolute;">block</div> <!-- flexbox item: block-level child --> <div id="item2" style="display:table">table</div> <!-- flexbox item: anonymous table wrapped around table-cell --> <div id="item3" style="display:table-cell">table-cell</div> <!-- flexbox item: anonymous block around inline content --> anonymous item 4 <!-- flexbox item: block-level child --> <div id="item5">block</div> <!-- flexbox item: anonymous block around inline content --> anonymous item 6.1 <span id="item6.1"> text 6.2 <div id="not-an-item6.3">block</div> text 6.4 </span> <!-- flexbox item: block-level replaced element --> <iframe id="item7"></iframe> <!-- flexbox item: inline-level replaced element --> <img id="item7" style="display:inline"> <!-- flexbox item: inline-level replaced element --> <button id="item8">button</button> <!-- flexbox item: inline-table --> <div id="item9" style="display:inline-table">table</div> <!-- flexbox item: floated inline, which changes to a block --> <span id="item10" style="float: left;">span</span> </div>
Notice that block element "not-an-item6.3" is not a separate flexbox item, because it is contained inside an inline element which is being wrapped into an anonymous flexbox item. Similarly, the block element "not-an-item1.5" is not a flexbox item, because it's absolutely positioned and thus out of flow.
Absolutely positioned children of a flexbox are not flexbox items, but they leave behind
"placeholders" in their normal position in the box tree. These
placeholders are anonymous inline boxes with a width and height of
‘0px
’, and they interact normally with
the flexbox layout algorithm. In particular, they'll trigger the creation
an anonymous flexbox item wrapper boxes, or join neighboring inline
elements in their anonymous flexbox item wrapper boxes.
The "static position" of an absolutely positioned child of a flexbox
(the position when the ‘top
’/‘right
’/‘bottom
’/‘left
’ properties are ‘auto
’), then, is the final position of their
corresponding placeholder, after flexbox layout has been performed.
Note: In most cases, this means that absolutely positioned
items will have no effect on flexbox layout, even if they force the
generation of an anonymous flexbox item wrapper, because those wrapper
items will also collapse to zero size and have no effect. The only
exception is when the flexbox has ‘flex-pack:justify
’, in which case the anonymous
flexbox item will cause there to be two packing spaces where there would
otherwise be only one, which will appear as a double-size space between
two "real" items.
A flexbox can be either single-line or
multi-line, depending on the ‘flex-wrap
’
property. A single-line flexbox lays out all of
its children in a single line, even if that would cause the flexbox to
overflow its bounds. A multi-line flexbox breaks
its flexbox items across multiple lines
to avoid overflowing, similar to how text is broken onto a new line when
it gets too wide to fit on the existing line. Every line contains at least
one flexbox item, unless the flexbox
itself is completely empty.
When additional lines are created, they are stacked in the flexbox in
the cross axis. Each line is completely
independent; flexible lengths and the ‘flex-pack
’ and ‘flex-align
’
properties only consider the items on a single line at a time. The main size of a line is the same as the main size of the flexbox's content box. The
cross size of a line depends on whether
the flexbox is single-line or multi-line: the cross size of the sole line in a single-line flexbox is the same as the cross size of the flexbox's content box,
while the cross size of a line in a multi-line flexbox is the minimum size
necessary to contain the flexbox items
on the line, after aligning them with ‘flex-align
’. The lines themselves are then
aligned within a flexbox with the ‘flex-line-pack
’ property.
This example shows four buttons that do not fit horizontally.
<style> #div1 { display: flexbox; flex-flow: row wrap; width: 300px; } button { flex:80px 1; } <style> <div id="div1"> <button id="button1">Elephant</button> <button id="button2">Tiger</button> <button id="button3">Antelope</button> <button id="button4">Wildebeest</button> </div>
The buttons are first set to their preferred widths, in this case 80
pixels. This will allow the first three buttons to fit in 240 pixels with
60 pixels left over of remaining space. Because the ‘flex-flow
’
property specifies a multi-line flexbox (due to the ‘wrap
’ keyword
appearing in its value), the flexbox will create an additional line to
contain the last button.
Flexibility is applied to each element, separately for each line. The first line has 60 pixels of remaining space and all of the buttons have the same flexibility, so each of the three buttons on that line will receive 20 pixels of extra width, ending up 100px wide. The remaining button is on a line of its own and will stretch to the entire width of the line, or 300 pixels.
If the box was resized, the buttons may rearrange onto different lines as necessary.
If the style rules in the example above were changed to the following:
#div1 { display: flexbox; flex-flow: row wrap; flex-pack: center; width: 300px; } button { flex:80px 1; max-width: 90px; }
Similar to the previous example, the first three buttons will fit on
the first line, and the last button will wrap onto a new line. However,
when the buttons attempt to flex they can only grow to 90px each, due to
their ‘max-width
’ property. This
leaves 30px of free space on the first line and 210px of free space on
the second line. Because ‘flex-pack
’ is set to ‘center
’,
the buttons will be centered on each line, with the free space split
equally on either side.
The first level of flexbox functionality is the ability to lay out a
flexbox's contents in any direction and in any order. This allows an
author to trivially achieve effects that would previously have required
complex or fragile methods, such as using the ‘float
’ property to lay out a horizontal
navigation bar (which then requires further effort with the ‘clear
’ property or others to make the elements
interact nicely with the rest of the page). This functionality is exposed
through the ‘flex-flow
’ and ‘flex-order
’
properties.
flex-direction
’ propertyName: | flex-direction |
---|---|
Value: | row | row-reverse | column | column-reverse |
Initial: | row |
Applies To: | flexboxes |
Inherited: | no |
Computed Value: | specified value |
Media: | visual |
Animatable: | no |
The ‘flex-direction
’ property specifies how flexbox items are placed in the flexbox,
by setting the direction of the flexbox's main
axis. This affects the direction that flexbox items are laid out
in, and the meaning of the ‘flex-pack
’ property.
flex-wrap
’ propertyName: | flex-wrap |
---|---|
Value: | nowrap | wrap | wrap-reverse |
Initial: | nowrap |
Applies To: | flexboxes |
Inherited: | no |
Computed Value: | specified value |
Media: | visual |
Animatable: | no |
The ‘flex-wrap
’ property controls whether the
flexbox is single-line or multi-line, and the direction of the cross axis, which affects the direction new
lines are stacked in and the meaning of the ‘flex-align
’,
‘flex-item-align
’, and ‘flex-line-pack
’ properties.
flex-flow
’ shorthandName: | flex-flow |
---|---|
Value: | <'flex-direction'> || <'flex-wrap'> |
Initial: | see individual properties |
Applies To: | flexboxes |
Inherited: | see individual properties |
Computed Value: | see individual properties |
Media: | visual |
Animatable: | no |
The ‘flex-flow
’ property is a shorthand for
setting the ‘flex-direction
’ and ‘flex-wrap
’
properties together.
Some examples of valid flows:
div { flex-flow: row; } /* Initial value. Main axis is inline, no wrap. */ | |
div { flex-flow: column wrap; } /* Main axis is block-direction and lines wrap in the inline direction. For an English page, the main axis is top-to-bottom and lines wrap to the right. */ | |
div { writing-mode: vertical-rl; flex-flow: column wrap-reverse; } /* Main axis is block direction (right to left). New lines wrap upwards. */ |
flex-order
’ propertyFlexbox items are, by default,
displayed and laid out in the same order as they appear in the source
document. The ‘flex-order
’ property may be used to change
this ordering.
Name: | flex-order |
---|---|
Value: | <number> |
Initial: | 0 |
Applies to: | flexbox items |
Inherited: | no |
Computed value: | specified value |
Media: | visual |
Animatable: | yes |
The ‘flex-order
’ property assigns flexbox items to ordinal groups.
Ordinal groups control the order in which flexbox items appear. A flexbox will lay
out its content starting from the lowest numbered ordinal group and going
up. Items with the same ordinal group are laid out in the order they
appear in the source document. ‘flex-order
’ has no effect on
stacking/layering; elements must still be drawn over/under each other
based on document order, ‘z-index
’, and other relevant means.
The following figure shows a simple tabbed interface, where the tab for the active pane is always in front:
This could be implemented with the following CSS (showing only the flexbox-relevant code):
.tabs { display: flexbox; } .tabs > .current { flex-order: -1; /* Lower than the default of 0 */ }
Many web pages have a similar shape in the markup, with a header on
top, a footer on bottom, and then a content area and one or two
additional columns in the middle. Generally, it's desirable that the
content come first in the page's source code, before the additional
columns. However, this makes many common designs, such as simply having
the additional columns on the left and the content area on the right,
difficult to achieve. This has been addressed in many ways over the
years, often going by the name "Holy Grail Layout" when there are two
additional columns. ‘flex-order
’ makes this trivial. For
example, take the following sketch of a page's code and desired layout:
<!DOCTYPE html> <header>...</header> <div id='main'> <article>...</article> <nav>...</nav> <aside>...</aside> </div> <footer>...</footer>
This layout can be easily achieved with Flexbox:
#main { display: flexbox; } #main > article { flex:1; flex-order: 2; } #main > nav { width: 200px; flex-order: 1; } #main > aside { width: 200px; flex-order: 3; }
As an added bonus, the columns will all be equal-height by default, and the main content will be as wide as necessary to fill the screen. Additionally, this can then be combined with media queries to switch to an all-vertical layout on narrow screens:
@media all and (max-width: 600px) { /* Too narrow to support three columns */ #main { flex-flow: column; } #main > article, #main > nav, #main > aside { /* Return them to document order */ flex-order: 0; width: auto; } }
(Further use of multiline flexboxes to achieve even more intelligent wrapping left as an exercise for the reader.)
flex
’ propertyThe defining aspect of flexbox layout is the ability to make the flexbox items "flex", altering their width
or height to fill the available space. This is done by using a ‘flex
’ property. A
flexbox distributes free space to its items proportional to their positive
flexibility, or shrinks them to prevent overflow proportional to their
negative flexibility.
Name: | flex |
---|---|
Value: | [ [ <pos-flex> <neg-flex>? ] || <preferred-size> ] | none |
Initial: | none |
Applies to: | flexbox items |
Inherited: | no |
Computed Value: | specified value |
Media: | visual |
Animatable: | yes |
The ‘flex
’
property specifies the parameters of a flexible
length: the positive and negative flexibility, and the preferred size.
<pos-flex>
and <neg-flex>
are
non-negative <number>s
. The <pos-flex>
component sets the positive
flexibility; if omitted, the positive flexibility defaults to
‘1
’. The <neg-flex>
component sets the negative
flexibility; if omitted, it defaults to ‘0
’.
The <preferred-size>
component sets the preferred size.
It can be set to any value that would be valid in the ‘width
’ or ‘height
’ property, except values that are not
applicable to a component value (of values defined in CSS2.1, only
'inherit' is not valid for <preferred-size>
; in
the future other values may be introduced that are not applicable as
length component). If omitted, the preferred size defaults to ‘0px
’. If set to ‘auto
’, the value of ‘width
’ or ‘height
’ (whichever is in parallel to main
axis) is used as preferred size.
If the <preferred-size>
is ‘0
’, it must be specified with a
unit (like ‘0px
’) to avoid ambiguity;
unitless zero will either be interpreted as as one of the flexibilities,
or is a syntax error.
The keyword ‘none
’ is equivalent
to "0 0 auto".
A ‘<flex>
’ value is
transitionable, by transitioning the preferred size, positive flexibility, and negative flexibility
independently. ‘<flex>
’ can also
transition to and from a <length>, by treating the length as if it were
a flexible length with a positive and negative flexibility of zero and a
preferred size of the length.
Flexibility allows elements to respond directly to the available space, optionally taking into account size of content:
<!DOCTYPE html> <style> div { display:flexbox; outline:1px solid silver; } p { flex:1 auto; margin:1em; background:gold; } </style> <div> <p>"flexing"</p> <p>allows the items to get bigger</p> <p>or</p> <p>smaller</p> </div>
After a flexbox's contents have finished their flexing and dimensions of
margin boxes of all flexbox items are finalized, they can be aligned in
both the main axis with ‘flex-pack
’ and
the cross axis with ‘flex-align
’ and
‘flex-item-align
’. These properties make
many common types of alignment trivial, including some things that were
very difficult in CSS 2.1, like horizontal and vertical centering.
flex-pack
’ propertyName: | flex-pack |
---|---|
Value: | start | end | center | justify | distribute |
Initial: | start |
Applies to: | flexboxes |
Inherited: | no |
Computed Value: | specified value |
Media: | visual |
Animatable: | no |
The ‘flex-pack
’ property aligns flexbox items in the main axis of the current line of the flexbox.
This is done after any flexible lengths and any auto
margins have been resolved. Typically it helps distribute extra free space
leftover when either all the flexbox
items on a line are inflexible, or are flexible but have reach
their maximum size, but it also exerts some control over the alignment of
items when they overflow the line.
start
’.
Otherwise, the main-start margin edge of
the first flexbox item on the line is
placed flush with the main-start edge of
the line, the main-end margin edge of the
last flexbox item on the line is
placed flush with the main-end edge of the
line, and the remaining flexbox items
on the line are distributed so that the empty space between any two
adjacent items is the same.
center
’. Otherwise, the flexbox items on the line are distributed
such that the empty space between any two adjacent flexbox items on the line is the same,
and the empty space before the first and after the last flexbox items on the line are half the
size of the other empty spaces.
An illustration of the five ‘flex-pack
’
keywords and their effects on a flexbox with three colored items.
flex-align
’ and ‘flex-item-align
’ propertiesName: | flex-align |
---|---|
Value: | start | end | center | baseline | stretch |
Initial: | stretch |
Applies to: | flexboxes |
Inherited: | no |
Computed Value: | specified value |
Media: | visual |
Animatable: | no |
Name: | flex-item-align |
---|---|
Value: | auto | start | end | center | baseline | stretch |
Initial: | auto |
Applies to: | flexbox items |
Inherited: | no |
Computed Value: | ‘auto ’ computes to parent's
‘flex-align ’; otherwise as specified
|
Media: | visual |
Animatable: | no |
Flexbox items can be aligned in the
cross axis of the current line of the
flexbox, similar to ‘flex-pack
’ but in the perpendicular
direction. ‘flex-align
’ sets the default alignment for
all of the flexbox's items, including anonymous flexbox items. ‘flex-item-align
’ allows this default
alignment to be overridden for individual flexbox items (for anonymous flexbox
items, ‘flex-item-align
’ always matches the value
of ‘flex-align
’ on their associated flexbox).
A value of ‘auto
’ for ‘flex-item-align
’ computes to the value of
‘flex-align
’ on the flexbox item's flexbox. The alignments are
defined as:
If the flexbox item's inline axis
is the same as the cross axis, this
value is identical to ‘start
’.
Otherwise, all flexbox items on the
line with an alignment of ‘baseline
’ that don't run afoul of the
previous paragraph are aligned such that their baselines align, and the
item with the largest distance between its baseline and its cross-start margin edge is placed flush
against the cross-start edge of the
line.
If the cross size property
of the flexbox item is ‘auto
’, it resolves to the length necessary to
make the cross size of the item's
margin box the same size as the line, while still respecting
‘min/max-width/height
’ constraints as
normal.
The cross-start margin edge of the flexbox item is placed flush with the cross-start edge of the line.
An illustration of the five ‘flex-align
’
keywords and their effects on a flexbox with four colored items.
By using a vertical flexbox and ‘flex-align
’, we can achieve behavior very
close to HTML's <center>
element:
<div> <p>foo foo foo foo</p> <p>bar bar<br>bar bar</p> <p>foo foo foo foo foo foo foo foo foo foo foo foo</p> </div> <style> div { display: flexbox; flex-flow: column; flex-align: center; width: 200px; } </style>
Margins on flexbox items can be set to 'auto', with effect very similar to auto margins in normal flow.
Auto margins can be used for simple alignment or for fine control.
Note that auto margins work consistently in both dimensions, so a simple markup like this
div { display:flexbox; width:4em; height:4em; background:silver; } p { margin:auto; } <div><p>OK</p></div>
will center the single child:
And this
div { display:flexbox; width:calc(100% - 4em); height:calc(100% - 4em); border: 1em solid blue; border-radius:50%; margin:auto; } div#demo { width:9em; height:9em; }
<div id="demo"><div><div></div></div></div>
will produce nested centered boxes:
flex-line-pack
’ propertyName: | flex-line-pack |
---|---|
Value: | start | end | center | justify | distribute | stretch |
Initial: | stretch |
Applies to: | multi-line flexboxes |
Inherited: | no |
Computed Value: | specified value |
Media: | visual |
Animatable: | no |
The ‘flex-line-pack
’ property aligns a
flexbox's lines within the flexbox when there is extra space in the cross axis, similar to how ‘flex-pack
’ aligns
individual items within the main axis:
start
’. Otherwise, the cross-start edge of the first line in the
flexbox is placed flush with the cross-start content edge of the flexbox,
the cross-end edge of the last line in
the flexbox is placed flush with the cross-end content edge of the flexbox, and
the remaining lines in the flexbox are distributed so that the empty
space between any two adjacent lines is the same.
center
’.
Otherwise, the lines in the flexbox are distributed such that the empty
space between any two adjacent lines is the same, and the empty space
before the first and after the last lines in the flexbox are half the
size of the other empty spaces.
start
’.
Otherwise, the free-space is split equally between all of the lines,
increasing their cross size.
Note: Only multi-line flexboxes ever have free space in the cross axis for lines to be aligned in, because in a single-line flexbox the sole line automatically stretches to fill the space.
An illustration of the ‘flex-line-pack
’ keywords and their
effects on a multi-line flexbox.
This section contains normative algorithms detailing the exact layout behavior of a flexbox and its contents. The algorithms here were designed to optimize readability and theoretical simplicity, and may not necessarily be the most efficient. Implementations may use whatever actual algorithms they wish, but must produce the same results as the algorithms described here.
This note will outline the general structure of the layout algorithm, before I go into the ugly details below, to aid in reading the relatively-long and complex algorithm.
flex-order
’.
flex-line-pack
’.
flex-align
’.
flex-pack
’.
This section is mainly intended for implementors. Authors writing web pages should generally be served well by the individual property descriptions, and do not need to read this section unless they have a deep-seated urge to understand arcane details of CSS layout.
To lay out a flexbox and its contents, follow these steps:
flex-order
’.
The items with the lowest (most negative) ‘flex-order
’
values are first in the ordering. If multiple items share a ‘flex-order
’
value, they're ordered by document order. This affects the order in which
the flexbox items generate boxes in the box-tree, and how the rest of
this algorithm deals with the items.
Otherwise layout the flexbox items using the shrink-to-fit algorithm:
In row-direction flexbox, shrink-to-fit size of a flex item along
the main axis is its ‘max-content
’, as defined in [CSS3-WRITING-MODES]
In column-direction flexbox, shrink-to-fit size of a flex item along the main axis is the extent in block direction that it gets when laid out using its 'fit-content' as its size in inline direction and using cross-size of flexbox content box as available measure, also as defined in [CSS3-WRITING-MODES].
In multi-line column-direction flexbox, cross-size of the whole flexbox content box is used as available measure, not the share of individual flexbox line, to avoid a circular dependency of line breaks on shrink-to-fit calculations.
Do not apply min/max-width/height constraints to the preferred size of flexible lengths - those constraints are handled elsewhere in this algorithm, and doing so will produce incorrect results.
If the flexbox is single-line, collect all the flexbox items into a single flexbox line.
If the flexbox is multi-line, group the flexbox items into multiple lines:
Determine the maximum line length, based on the main size of the flexbox. The maximum line length is main size of the flexbox's content box, constrained by the min and max main size constraints of the flexbox. If the main size depends on the flexbox's content, then:
min-content
’, the maximum
line length is the flexbox's minimum main size, constrained by the
min and max main size constraints of the flexbox.
max-content
’, the maximum
line length is infinity, constrained by the min and max main size
constraints of the flexbox.
fit-content
’, the maximum
line length is the greater of the flexbox's min size constraint and
the smaller of the flexbox's max size constraint and the available
space.
flex-pack
’.
flex-item-align
’ of ‘baseline
’. Find the maximum of the
distances from their baseline to the cross-start edge of their margin
box, and the maximum of the distances from their baseline to the
cross-end edge of their margin box. Sum these two values.
Determine the final cross size of each flexbox item.
flex-item-align:stretch'' and its preferred cross
size is 'auto' and its margin-box cross size is smaller than
cross-size of its flexbox line, its final cross size is set so that its
margin-box cross size is equal to cross-size of its flexbox line.
To resolve the flexible lengths of the items within a flexbox line:
for the rest of the algorithm use flexibility with the sign determined in this step
For the purposes of this algorithm, measurements are considered definite if their precise value is known before the start of the algorithm. For example, sizes in pixels or inches are always definite; 'auto' and percent may or may not be definite, which depends on how these values were handled before this algorithm is applied.
Flexboxes can break across pages between items, between lines of items (in multi-line mode) and inside items, as long as 'break-' property allow that. All 'break-' properties are supported on flexbox, on flexbox items and inside flexbox items.
The following breaking rules refer to fragmentation container as “page”. The same rules apply to any other fragmenters. Change “page” to the appropriate fragmenter type as needed.
Breaks in and around flexbox are determined as follows:
If as a result of this adjustment block-direction size of flexbox becomes negative, it is set to zero.
If flexbox main-axis size is definite, flexbox algorithm must be run first, without constraints of available space on page and using all content of the flexbox. Otherwise flexbox items use the preferred size, adjusted for min/max.
If page size varies and flexbox size depends on page size, this step has to be repeated on each page, again with the whole content of the flexbox.
Items that fit on a page completely or partially are aligned according to ‘flex-pack’ property, independently from the rest of flexbox content.
Note that flexible lengths are not recalculated on each page, even if there is additional free space.
It is the intent of this spec that column-direction single-line flexbox paginates very similarly to block flow. As a test of the intent, a flexbox with "flex-pack:start" and no flexible items should paginate identically to a block with non-floating children with same content, same used size and same used margins. This rule is simplified and not normative, but if there is any difference it should be noted here.
After the break, continuation of overflow items may overlap with other items and/or content after the flexbox. It is undesirable but there is no good resolution for this spacial conflict and this outcome is similar to effect of "overflow:visible" elsewhere.
Main-axis space distribution in each line is done based on complete content of the flexbox and without space constraint in block direction.
If page size varies and flexbox size depends on page size, this step needs to be repeated on each page.
If 'flex-line-pack' is 'stretch' and flexbox size in block direction is definite, layout of the complete flexbox has to be done first to determine line heights. Layout is done as usual for non-paginated case, but accounting for forced breaks.
If page size varies, this step may have to be redone, again with complete content; special consideration should be to be given to line breaks to ensure that item at the start of current page is also at the start of a line in this hypothetical layout. This specification currently doesn't define how exactly to achieve that.
'Flex' property is currently defined as shorthand only, combining three values. When queried from OM, it is seen as a string with up to three values, which can be very complicated to deal with properly.
There should be separate properties to access flexibility components in OM, such as:
[Supplemental] interface "CSSStyleDeclaration { attribute DOMString? flexPositive; attribute DOMString? flexNegative; attribute DOMString? flexPreferredSize; };See Bug-16145
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 Flexbox Layout Module is defined for three conformance classes:
A style sheet is conformant to CSS Flexbox Layout Module 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 Flexbox Layout Module if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Flexbox Layout Module 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 Flexbox Layout Module 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 implementers 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.
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.
Thanks for feedback and contributions to Andrew Fedoniouk, Arron Eicholz, James Elmore, Ben Horst, Boris Zbarsky, Brad Kemper, Brian Heuston, Christian Stockwell, Christoph Päper, Daniel Holbert, Erik Anderson, Eugene Veselov, Fantasai, John Jansen, Markus Mielke, Ning Rogers, Ojan Vafai, Peter Salas, Phil Cupp, Robert O'Callahan, Rossen Atanassov, Shinichiro Hamaji, Tony Chang.
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
display | flexbox | inline-flexbox | |||||
flex | [ [ <pos-flex> <neg-flex>? ] || <preferred-size> ] | none | none | flexbox items | no | specified value | visual |
flex-align | start | end | center | baseline | stretch | stretch | flexboxes | no | specified value | visual |
flex-direction | row | row-reverse | column | column-reverse | row | flexboxes | no | specified value | visual |
flex-flow | <'flex-direction'> || <'flex-wrap'> | see individual properties | flexboxes | see individual properties | see individual properties | visual |
flex-item-align | auto | start | end | center | baseline | stretch | auto | flexbox items | no | ‘auto’ computes to parent's ‘flex-align’; otherwise as specified | visual |
flex-line-pack | start | end | center | justify | distribute | stretch | stretch | multi-line flexboxes | no | specified value | visual |
flex-order | <number> | 0 | flexbox items | no | specified value | visual |
flex-pack | start | end | center | justify | distribute | start | flexboxes | no | specified value | visual |
flex-wrap | nowrap | wrap | wrap-reverse | nowrap | flexboxes | no | specified value | visual |
, ’‘auto
’‘">8.2.