The specification describes a CSS box model
optimized for user interface design. In the flex layout model, the
children of a flex container 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.
Status of this document
This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as 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…”
The CR period will last at least until 20 March 2013. At the time of
publication, no test suite and implementation report have yet been made.
They will be made available from the CSS test
suites page. See the section “CR exit
criteria” for details.
See the section “Changes” for changes made to
this specification since the last Working Draft.
The following features are at-risk:
Calculation of the static position of absolutely-positioned flex
items.
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 2D data in a tabular format
positioned layout, designed for very explicit positioning without much
regard for other elements in the document
This module introduces a new layout mode, flex
layout, which is designed for laying out more complex applications
and webpages.
1.1. Overview
This section is not normative.
Flex layout is superficially similar to block layout. It lacks many of
the more complex text- or document-centric properties that can be used in
block layout, such as floats and columns. In return it gains
simple and powerful tools for distributing space and aligning content in
ways that webapps and complex web pages often need. The contents of a flex
container:
can be laid out in any flow
direction (leftwards, rightwards, downwards, or even upwards!)
can have their display order reversed or rearranged at the style layer (i.e., visual
order can be independent of source and speech order)
can be laid out linearly along a single (main) axis or wrapped into multiple lines along a
secondary (cross)
axis
Here's an example of a catalog where each item has a title, an photo,
a description, and a purchase button. The designer's intention is that
each entry has the same overall size, that the photo be above the text,
and that the purchase buttons aligned at the bottom, regardless of the
length of the item's description. Flex layout makes many aspects of this
design easy:
The catalog uses flex layout to lay out rows of items horizontally,
and to ensure that items within a row are all equal-height. Each entry
is then itself a column flex container, laying out its contents
vertically.
Within each entry, the source document content is ordered logically
with the title first, followed by the description and the photo. This
provides a sensible ordering for speech rendering and in non-CSS
browsers. For a more compelling visual presentation, however, ‘order’ is used to pull
the image up from later in the content to the top, and ‘align-self’ is used
to center it horizontally.
An ‘auto’
margin above the purchase button forces it to the bottom within each
entry box, regardless of the height of that item's description.
<section id='deals'>
<section class='sale-item'>
<h1>Computer Starter Kit</h1>
<p>This is the best computer money can buy, if you don't have much money.
<ul>
<li>Computer
<li>Monitor
<li>Keyboard
<li>Mouse
</ul>
<img src='images/computer.jpg'
alt='You get: a white computer with matching peripherals.'>
<button>BUY NOW</button>
</section>
<section class='sale-item'>
…
</section>
…
</section>
Computer Starter Kit
This is the best computer money can buy, if you don't have much
money.
Computer
Monitor
Keyboard
Mouse
Printer
Only capable of printing ASCII art.
Paper and ink not included.
An example rendering of the code above.
1.2. Module interactions
This module extends the definition of the ‘display’ property [CSS21], adding a new block-level and
new inline-level display type, and defining a new type of formatting
context along with properties to control its layout. None of the
properties defined in this module apply to the ‘::first-line’ or ‘::first-letter’ pseudo-elements.
1.3. Values
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 [CSS3VAL], when
combined with this module, expands the definition of the
<length> 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.
2. Flex Layout Box Model and
Terminology
An element with ‘display:flex’ or ‘display:inline-flex’ is a flex container. Children of a flex
container are called flex items and are
laid out using the flex layout model.
Unlike block and inline layout, whose layout calculations are biased to
the block
and inline flow directions, flex layout is biased to the flex flow directions. To make it easier to
talk about flex layout, this section defines a set of flex flow–relative
terms. The ‘flex-flow’ value determines how these terms
map to physical directions (top/right/bottom/left), axes
(vertical/horizontal), and sizes (width/height).
An illustration of the various directions and sizing terms as applied to a
‘row’ flex
container.
main axis
main dimension
The main axis of a flex container is the
primary axis along which flex items are
laid out. It extends in the main dimension.
main-start
main-end
The flex items are placed within the
container starting on the main-start side and
going toward the main-end side.
main size
main size property
A flex item's width or height,
whichever is in the main dimension,
is the item's main size. The flex item's main size property is either the ‘width’ or ‘height’ property, whichever is in the main dimension.
cross axis
cross dimension
The axis perpendicular to the main
axis is called the cross axis. It
extends in the cross dimension.
cross-start
cross-end
Flex lines are filled with items and placed into the container
starting on the cross-start side of the flex
container and going toward the cross-end side.
cross size
cross size property
The width or height of a flex item,
whichever is in the cross
dimension, is the item's cross size. The
cross size property is whichever of
‘width’ or ‘height’ that is in the cross dimension.
This value causes an element to generate a block-level flex container box.
‘inline-flex’
This value causes an element to generate an inline-level flex container box.
A flex container establishes a new flex formatting context for its contents.
This is the same as establishing a block formatting context, except that
flex layout is used instead of block layout: floats do not intrude into
the flex container, and the flex container's margins do not collapse with
the margins of its contents. Flex
containers form a containing block for their contents exactly
like block containers do. [CSS21] The ‘overflow’ property applies to flex containers.
Flex containers are not block containers, and so some properties that
were designed with the assumption of block layout don't apply in the
context of flex layout. In particular:
all of the ‘column-*’ properties in the
Multicol module have no effect on a flex container.
‘float’ and ‘clear’ have no effect on a flex item. (However, the ‘float’ property still affects the computed value
of ‘display’ on children of a flex
container, as this occurs beforeflex items are determined.)
‘vertical-align’ has no effect on a
flex item.
the ‘::first-line’ and ‘::first-letter’ pseudo-elements do not apply to flex containers.
If an element's specified ‘display’ is
‘inline-flex’ and the element is floated or
absolutely positioned, the computed value of ‘display’ is ‘flex’. The table in CSS 2.1 Chapter
9.7 is thus amended to contain an additional row, with ‘inline-flex’ in
the "Specified Value" column and ‘flex’ in the "Computed Value" column.
4. Flex Items
The contents of a flex container
consists of zero or more flex
items: each in-flow child of a flex
container becomes a flex item, and
each contiguous run of text that is directly contained inside a flex container is wrapped in an
anonymous flex item element. However, an
anonymous flex item that contains only white
space is not rendered, as if it were ‘display:none’.
A flex item establishes a new
formatting context for its contents. The type of this formatting context
is determined by its ‘display’ value, as
usual. The computed ‘display’ of a flex item is determined by applying the table
in CSS 2.1
Chapter 9.7. However, flex items are flex-level boxes, not block-level
boxes: they participate in their container's flex formatting context, not
in a block formatting context.
The ‘display’ computation on
flex items as defined here is expected to be superseded by a future
specification that defines a new ‘display’
value specific to flex items.
Some values of ‘display’ trigger the
generation of anonymous boxes. For example, a misparented ‘table-cell’ child is fixed up by generating
anonymous ‘table’ and ‘table-row’ elements around it. [CSS21] This fixup must occur
before a flex container's children are promoted to flex items. For example, given two contiguous
child elements with ‘display:table-cell’, an
anonymous table wrapper box around them becomes the flex item.
Future display types may generate anonymous containers (e.g.
ruby) or otherwise mangle the box tree (e.g. run-ins). It is intended that
flex item determination run after these operations.
On a flex item with ‘display:
table’, the table wrapper box becomes the flex item, and the ‘order’ and ‘align-self’ properties apply to it. The
contents of any caption boxes contribute to the calculation of the table
wrapper box's min-content and max-content sizes. However, like ‘width’ and ‘height’, the ‘flex’ longhands apply to the table box as
follows: the flex item's final size
is calculated by performing layout as if the distance between the table
wrapper box's edges and the table box's content edges were all part of the
table box's border+padding area, and the table box were the flex item.
4.1. Absolutely-Positioned
Flex Children
An absolutely-positioned child element of a flex container does not participate in
flex layout beyond the reordering step.
However, if both ‘left’ and ‘right’ or both ‘top’ and ‘bottom’
are ‘auto’, then the used value of those
properties are computed from its static
position, as follows:
If both ‘left’ and ‘right’ are ‘auto’, the
absolutely-positioned child must be positioned so that its main-start or cross-start edge (whichever is in the
horizontal axis) is aligned with the static position. If both
‘top’ and ‘bottom’ are ‘auto’, the
absolutely-positioned child must be positioned so that its main-start or cross-start edge (whichever is in the
vertical axis) is aligned with the static position.
If there is no preceding flex item or the preceding flex item and the
subsequent flex item are on the same flex line, the static position is the outer
main-start edge of that flex item.
Otherwise, the static
position is the cross-start edge of the first flex
line.
The static position is intended to more-or-less match the
position of an anonymous 0×0 in-flow ‘flex-start’-aligned flex item that participates in
flex layout, the primary difference being that any packing spaces due to
‘justify-content: space-around’ or ‘justify-content: space-between’ are suppressed around
the hypothetical item: between it and the next item if there is a real
item after it, else between it and the previous item (if any) if there
isn't.
4.2. Flex Item Margins and
Paddings
The margins of adjacent flex items do
not collapse. Auto margins absorb extra space in the corresponding
dimension and can be used for alignment and to push adjacent flex items
apart; see Aligning with ‘auto’ margins.
Percentage margins and paddings on flex
items are always resolved against their respective dimensions;
unlike blocks, they do not always resolve against the inline dimension of
their containing block.
4.3. Flex Item Z-Ordering
Flex items paint exactly the same as
block-level elements, except that ‘order’-modified document order is used in
place of raw document order, and ‘z-index’ values other than ‘auto’ create a stacking context even if ‘position’ is ‘static’.
Note: Descendants that are positioned outside a flex item
still participate in any stacking context established by the flex item.
4.4. Collapsed Items
Specifying ‘visibility:collapse’ on a flex
item causes it to become a collapsed flex item, producing
an effect similar to ‘visibility:collapse’ on a
table-row or table-column: the collapsed element is removed from rendering
entirely, but leaves behind a "strut" that keeps the flex line's
cross-size stable. Thus, if a flex container has only one flex line,
dynamically collapsing or uncollapsing items is guaranteed to have no
effect on the flex container's cross size
and won't cause the rest of the page's layout to "wobble". Flex line
wrapping is re-done after collapsing, however, so the cross-size
of a flex container with multiple lines might or might not change.
Though collapsed flex items aren't rendered, they do appear in the formatting
structure. Therefore, unlike on ‘display:none’ items [CSS21], effects that depend on an
element appearing in the formatting structure (like incrementing counters
or running animations and transitions) still operate on collapsed items.
In the following example, a sidebar is sized to fit its content.
‘Visibility: collapse’ is used to dynamically
hide parts of a navigation sidebar without affecting its width, even
though the widest item (“Architecture”) is in a collapsed section.
Sample rendering for example code below
Interesting Stuff to Read
<style>
@media (min-width: 60em) {
/* two column layout only when enough room (relative to default text size) */
header + div { display: flex; }
#main {
flex: 1; /* Main takes up all remaining space */
order: 1; /* Place it after (to the right of) the navigation */
min-width: 12em; /* Optimize main content area sizing */
}
}
/* menu items use flex layout so that visibility:collapse will work */
nav > ul > li {
display: flex;
flex-flow: column;
}
/* dynamically collapse submenus when not targetted */
nav > ul > li:not(:target):not(:hover) > ul {
visibility: collapse;
}
</style>
…
</header>
<div>
<article id="main">
Interesting Stuff to Read
</article>
<nav>
<ul>
<li id="nav-about"><a href="#nav-about">About</a>
…
<li id="nav-projects"><a href="#nav-projects">Projects</a>
<ul>
<li><a href="…">Art</a>
<li><a href="…">Architecture</a>
<li><a href="…">Music</a>
</ul>
<li id="nav-interact"><a href="#nav-interact">Interact</a>
…
</ul>
</nav>
</div>
<footer>
…
To compute the size of the strut, flex layout is first performed with
all items uncollapsed, and then re-run with each collapsed item replaced
by a strut that maintains the original cross-size of the item's original
line. See the Flex Layout Algorithm for
the normative definition of how ‘visibility:collapse’ interacts with flex layout.
Note that using ‘visibility:collapse’ on any flex items will cause the
flex layout algorithm to repeat partway through, re-running the most
expensive steps. It's recommended that authors continue to use ‘display:none’ to hide items if the items will not be
dynamically collapsed and uncollapsed, as that is more efficient for the
layout engine. (Since only part of the steps need to be repeated when
‘visibility’ is changed, however, ‘visibility: collapse’ is still recommended for dynamic
cases.)
5. Ordering and Orientation
The contents of a flex container can be laid out 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 hacks
using the ‘float’ and ‘clear’ properties. This functionality is exposed
through the ‘flex-direction’, ‘flex-wrap’, and ‘order’ properties.
The reordering capabilities of flex layout intentionally
affect only the visual rendering, leaving speech order and
navigation based on the source order. This allows authors to manipulate
the visual presentation while leaving the source order intact for non-CSS
UAs and for linear models such as speech and sequential navigation. See Reordering and Accessibility and the Flex Layout Overview for examples that use this
dichotomy to improve accessibility.
Authors must not use these techniques as a substitute for
correct source ordering, as that can ruin the accessibility of the
document.
The ‘flex-direction’ property specifies how flex items are placed in the flex container,
by setting the direction of the flex container's main axis. This determines the direction that
flex items are laid out in.
‘row’
The flex container's main axis has
the same orientation as the inline
axis of the current writing mode. The main-start and main-end directions are equivalent to the start and end directions,
respectively, of the current writing mode.
The flex container's main axis has
the same orientation as the block axis
of the current writing mode. The main-start and main-end directions are equivalent to the before and after
directions, respectively, of the current writing mode.
The reverse values do not reverse box ordering; like
‘writing-mode’ and ‘direction’ [CSS3-WRITING-MODES],
they only change the direction of flow. Painting order, speech order, and
sequential navigation orders are not affected.
The ‘flex-wrap’ property controls whether the flex
container is single-line or multi-line, and the direction of the
cross-axis, which determines the direction new lines are stacked
in.
‘nowrap’
The flex container is single-line.
The cross-start direction is equivalent
to either the start or
before
direction of the current writing mode, whichever is in the cross axis, and the cross-end direction is the opposite
direction of cross-start.
‘wrap’
The flex container is multi-line.
The cross-start direction is equivalent
to either the start or
before
direction of the current writing mode, whichever is in the cross axis, and the cross-end direction is the opposite
direction of cross-start.
The ‘flex-flow’
property is a shorthand for setting the ‘flex-direction’ and ‘flex-wrap’ properties,
which together define the flex container's main and cross axes.
Some examples of valid flows in an English (left-to-right, horizontal
writing mode) document:
div { flex-flow: row; }
/* Initial value. Main-axis is
inline, no wrap. */
div { flex-flow: column wrap; }
/* Main-axis is block-direction (top to bottom)
and lines wrap in the inline direction (rightwards). */
div { flex-flow: row-reverse wrap-reverse; }
/* Main-axis is the opposite of inline direction
(right to left). New lines wrap upwards. */
Note that the ‘flex-flow’ directions are writing-mode
sensitive. In vertical Japanese, for example, a ‘row’ flex container
lays out its contents from top to bottom, as seen in this example:
Flex items are, by default, displayed
and laid out in the same order as they appear in the source document. The
‘order’ property can
be used to change this ordering.
flex items and absolutely-positioned flex container children
Inherited:
no
Computed value:
specified value
Media:
visual
Animatable:
yes
Canonical Order:
per
grammar
The ‘order’
property controls the order in which flex items appear within their flex
container, by assigning them to ordinal groups.
A flex container 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. This also
affects the painting
order[CSS21],
exactly as if the elements were reordered in the document.
The following figure shows a simple tabbed interface, where the tab for
the active pane is always first:
This could be implemented with the following CSS (showing only the
relevant code):
.tabs {
display: flex;
}
.tabs > * {
min-width: min-content;
/* Prevent tabs from getting too small for their content. */
}
.tabs > .current {
order: -1; /* Lower than the default of 0 */
}
Unless otherwise specified by a future specification, this property has
no effect on elements that are not flex
items.
5.4.1. Reordering and
Accessibility
The ‘order’
property does not affect ordering in non-visual media (such as speech). Likewise, ‘order’ does not affect the
default traversal order of sequential navigation modes (such as cycling
through links, see e.g. ‘nav-index’ [CSS3UI] or tabindex[HTML40]). Authors
must use ‘order’ only for visual, not logical,
reordering of content; style sheets that use ‘order’ to perform logical reordering are
non-conforming.
This is so that non-visual media and non-CSS UAs, which
typically present content linearly, can rely on a logical source order,
while ‘order’ is used
to tailor the visual order. (Since visual perception is two-dimensional
and non-linear, the desired visual order is not always logical.)
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. ‘order’ makes this trivial. For example, take
the following sketch of a page's code and desired layout:
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 */
order: 0; width: auto;
}
}
(Further use of multi-line flex containers to achieve even more
intelligent wrapping left as an exercise for the reader.)
A single-line flex container lays out all
of its children in a single line, even if that would cause its contents
to overflow.
A multi-line flex container breaks its flex items across multiple lines, similar to
how text is broken onto a new line when it gets too wide to fit on the
existing line. When additional lines are created, they are stacked in the
flex container along the cross axis
according to the ‘flex-wrap’ property. Every line contains at
least one flex item, unless the flex
container itself is completely empty.
This example shows four buttons that do not fit horizontally.
Since the container is 300px wide, only three of the items fit onto a
single line. They take up 240px, with 60px left over of remaining space.
Because the ‘flex-flow’ property specifies a multi-line flex container (due to the ‘wrap’ keyword
appearing in its value), the flex container will create an additional
line to contain the last item. An example rendering of
the multi-line flex container.
Once content is broken into lines, each line is laid out independently;
flexible lengths and the ‘justify-content’ and ‘align-self’
properties only consider the items on a single line at a time.
When a flex container has multiple lines, the cross size of each line is the minimum size
necessary to contain the flex items on the
line (after aligment due to ‘align-self’), and the lines are aligned within
the flex container with the ‘align-content’ property. When a flex container
(even a multi-line one) has only one
line, the cross size of the line is the
cross size of the flex container, and
‘align-content’ has no effect. The main size of a line is always the same as the
main size of the flex container's content
box.
Here's the same example as the previous, except that the flex items
have all been given ‘flex: auto’. The first
line has 60px of remaining space, and all of the items have the same
flexibility, so each of the three items on that line will receives 20px
of extra width, ending up 100px wide. The remaining item is on a line of
its own and will stretch to the entire width of the line, or 300px.
A rendering
of the same as above, but with the items all given ‘flex: auto’.
7. Flexibility
The defining aspect of flex layout is the ability to make the flex items "flex", altering their width or
height to fill the available space. This is done with the ‘flex’ property. A flex
container distributes free space to its items proportional to their flex grow factor, or shrinks them to
prevent overflow proportional to their flex shrink factor.
This <number> component sets ‘flex-grow’ longhand and specifies the flex grow factor, which determines how much
the flex item will grow relative to the
rest of the flex items in the flex
container when positive free space is distributed. When omitted, it is
set to ‘1’.
This <number> component sets ‘flex-shrink’ longhand and specifies the flex shrink factor, which determines how
much the flex item will shrink relative
to the rest of the flex items in the
flex container when negative free space is distributed. When omitted, it
is set to ‘1’. The flex shrink factor is multiplied
by the flex basis when distributing
negative space.
This component, which takes the same values as the ‘width’ property, sets the ‘flex-basis’
longhand and specifies the flex basis: the initial main
size of the flex item, before
free space is distributed according to the flex factors. When omitted
from the ‘flex’
shorthand, its specified value is the length zero.
If the specified ‘flex-basis’ is ‘auto’, the used
flex basis is the value of the
element's main size property.
(This can itself be the keyword ‘auto’, which
sizes the element based on its contents.) A diagram
showing the difference between "absolute" flex (starting from a basis of
zero) and "relative" flex (starting from a basis of the item's content
size). The three items have flex factors of ‘1’, ‘1’, and ‘2’, respectively.
Note that the initial values of ‘flex-grow’ and ‘flex-basis’
are different from their defaults when omitted in the ‘flex’ shorthand. This so
that the ‘flex’
shorthand can better accommodate the most common
cases.
A unitless zero that is not already preceded by two flex factors must
be interpreted as a flex factor. To avoid misinterpretation or invalid
declarations, authors must specify a zero <flex-basis>
component with a unit or precede it by two flex factors.
The list below summarizes the effects of the most common ‘flex’ values:
‘flex: 0 auto’
‘flex: initial’
Equivalent to ‘flex: 0 1 auto’. (This is
the initial value.) Sizes the item based on the ‘width’/‘height’
properties. (If the item's main size
property computes to ‘auto’, this will
size the flex item based on its contents.) Makes the flex item inflexible
when there is positive free space, but allows it to shrink to its
min-size when there is insufficient space. The alignment abilities or ‘auto’ margins can be
used to align flex items along the main
axis.
‘flex: auto’
Equivalent to ‘flex: 1 1 auto’. Sizes the
item based on the ‘width’/‘height’ properties, but makes them fully
flexible, so that they absorb any free space along the main axis. If all items are either ‘flex: auto’, ‘flex:
initial’, or ‘flex: none’, any
positive free space after the items have been sized will be distributed
evenly to the items with ‘flex: auto’.
‘flex: none’
Equivalent to ‘flex: 0 0 auto’. This value
sizes the item according to the ‘width’/‘height’
properties, but makes the flex item fully inflexible. This is similar to
‘initial’, except
that flex items are not allowed to shrink, even in overflow situations.
‘flex: <positive-number>’
Equivalent to ‘flex: <positive-number> 1
0px’. Makes the flex item flexible and sets the flex basis to zero, resulting in an item
that receives the specified proportion of the free space in the flex
container. If all items in the flex container use this pattern, their
sizes will be proportional to the specified flex factor.
Note that, because flex items typically can shrink, it is
often recommended that a minimum main size be set (with ‘min-width’ or ‘min-height’) to prevent them from shrinking too
small to fit their contents. This problem is especially common with
horizontal navigation or toolbars.
nav li { flex: 1; min-width: min-content; /* Don't overflow */}
7.3. Components of
Flexibility
Individual components of flexibility can be controlled by independent
longhand properties.
Authors are encouraged to control flexibility using the
‘flex’ shorthand
rather than with component properties, as the shorthand correctly resets
any unspecified components to accommodate common
uses.
Except for ‘auto’, which retrieves the value of the main size property, ‘flex-basis’
is resolved the same way as ‘width’ in horizontal writing modes [CSS21]: percentage
values of ‘flex-basis’ are resolved against the flex
item's containing block, i.e. its flex container, and if that containing
block's size is indefinite, the result is undefined. Similarly, ‘flex-basis’
determines the size of the content box, unless otherwise specified such as
by ‘box-sizing’ [CSS3UI].
8. Alignment
After a flex container's contents have finished their flexing and the
dimensions of all flex items are finalized, they can then be aligned
within the flex container.
The ‘margin’ properties can be used to
align items in a manner similar to, but more powerful than, what margins
can do in block layout. Flex items also
respect the alignment properties from the Box Alignment spec, which allow
easy keyword-based alignment of items in both the main axis and cross
axis. 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.
While the alignment properties are defined in the Box
Alignment spec, Flexible Box Layout reproduces the definitions of the
relevant ones here so as to not create a normative dependency that may
slow down advancement of the spec. These properties apply only to flex
layout until Box Alignment is finished and defines their effect for other
layout modes.
8.1. Aligning with ‘auto’ margins
This section is non-normative. The normative definition of how
margins affect flex items is in the Flex
Layout Algorithm section.
Auto margins on flex items have an effect very similar to auto margins
in block flow:
During calculations of flex bases and flexible lengths, auto margins
are treated as ‘0’.
Prior to alignment via ‘justify-content’ and ‘align-self’, any
positive free space is distributed to auto margins in that dimension.
Overflowing elements ignore their auto margins and overflow in the
end/after direction.
Note that, if free space is distributed to auto margins, the
alignment properties will have no effect in that dimension because the
margins will have stolen all the free space left over after flexing.
One use of ‘auto’ margins in the main axis
is to separate flex items into distinct "groups". The following example
shows how to use this to reproduce a common UI pattern - a single bar of
actions with some aligned on the left and others aligned on the right.
Sample rendering of the code below.
<style>
nav > ul {
display: flex;
}
nav > ul > li {
min-width: min-content;
/* Prevent items from getting too small for their content. */
}
nav > ul > #login {
margin-left: auto;
}
</style>
<nav>
<ul>
<li><a href=/about>About</a>
<li><a href=/projects>Projects</a>
<li><a href=/interact>Interact</a>
<li id='login'><a href=/login>Login</a>
</ul>
</nav>
The figure below illustrates the difference in cross-axis alignment in
overflow situations between using ‘auto’ margins and using the alignment properties.
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The items in the figure on the left are centered with
margins, while those in the figure on the right are centered with ‘align-self’. If this
column flex container was placed against the left edge of the page, the
margin behavior would be more desirable, as the long item would be fully
readable. In other circumstances, the true centering behavior might be
better.
flex-start | flex-end | center | space-between | space-around
Initial:
flex-start
Applies to:
flex containers
Inherited:
no
Computed value:
specified value
Media:
visual
Animatable:
no
Canonical Order:
per
grammar
The ‘justify-content’ property aligns flex items along the main axis of the current line of the flex
container. 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 flex items on a line are inflexible, or are
flexible but have reached their maximum size. It also exerts some control
over the alignment of items when they overflow the line.
‘flex-start’
Flex items are packed toward the
start of the line. The main-start margin
edge of the first flex item on the line
is placed flush with the main-start edge
of the line, and each subsequent flex
item is placed flush with the preceding item.
‘flex-end’
Flex items are packed toward the end
of the line. The main-end margin edge of
the last flex item is placed flush with
the main-end edge of the line, and each
preceding flex item is placed flush with
the subsequent item.
‘center’
Flex items are packed toward the
center of the line. The flex items on the
line are placed flush with each other and aligned in the center of the
line, with equal amounts of empty space between the main-start edge of the line and the first
item on the line and between the main-end
edge of the line and the last item on the line. (If the leftover
free-space is negative, the flex items
will overflow equally in both directions.)
‘space-between’
Flex items are evenly distributed in
the line. If the leftover free-space is negative or there is only a
single flex item on the line, this value
is identical to ‘flex-start’. Otherwise, the main-start margin edge of the first flex item on the line is placed flush with
the main-start edge of the line, the main-end margin edge of the last flex item on the line is placed flush with
the main-end edge of the line, and the
remaining flex items on the line are
distributed so that the empty space between any two adjacent items is the
same.
‘space-around’
Flex items are evenly distributed in
the line, with half-size spaces on either end. If the leftover free-space
is negative or there is only a single flex
item on the line, this value is identical to ‘center’.
Otherwise, the flex items on the line are
distributed such that the empty space between any two adjacent flex items on the line is the same, and the
empty space before the first and after the last flex items on the line are half the size of
the other empty spaces.
An illustration of the five ‘justify-content’ keywords and their effects
on a flex container with three colored items.
‘auto’ computes to parent's ‘align-items’;
otherwise as specified
Media:
visual
Animatable:
no
Canonical Order:
per
grammar
Flex items can be aligned in the cross axis of the current line of the flex
container, similar to ‘justify-content’ but in the perpendicular
direction. ‘align-items’ sets the default alignment for
all of the flex container's items, including anonymous flex items. ‘align-self’ allows this default alignment to
be overridden for individual flex items.
(For anonymous flex items, ‘align-self’ always matches the value of ‘align-items’ on
their associated flex container.)
If either of the flex item's cross-axis
margins are ‘auto’, ‘align-self’ has no
effect.
A value of ‘auto’ for ‘align-self’ computes to the value of ‘align-items’ on the
element's parent, or ‘stretch’ if the element has no parent. The
alignments are defined as:
The flex item's margin box is
centered in the cross axis within the
line. (If the cross size of the flex
line is less than that of the flex item,
it will overflow equally in both directions.)
Otherwise, it participates in baseline
alignment: all participating flex
items on the line 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.
‘stretch’
If the cross size property
of the flex item computes to ‘auto’, its used value is the length necessary to make
the cross size of the item's margin box
as close to the same size as the line as possible, while still
respecting the constraints imposed by ‘min/max-width/height’.
Note that if the flex container's height is constrained
this value may cause the contents of the flex
item to overflow the item.
The ‘align-content’ property aligns a flex
container's lines within the flex container when there is extra space in
the cross-axis, similar to how ‘justify-content’ aligns individual items
within the main-axis. Note, this property has no effect when the
flex container has only a single line. Values have the following meanings:
‘flex-start’
Lines are packed toward the start of the flex container. The cross-start edge of the first line in the
flex container is placed flush with the cross-start edge of the flex container,
and each subsequent line is placed flush with the preceding line.
‘flex-end’
Lines are packed toward the end of the flex container. The cross-end edge of the last line is placed
flush with the cross-end edge of the flex
container, and each preceding line is placed flush with the subsequent
line.
‘center’
Lines are packed toward the center of the flex container. The lines
in the flex container are placed flush with each other and aligned in the
center of the flex container, with equal amounts of empty space between
the cross-start content edge of the
flex container and the first line in the flex container, and between the
cross-end content edge of the flex
container and the last line in the flex container. (If the leftover
free-space is negative, the lines will overflow equally in both
directions.)
‘space-between’
Lines are evenly distributed in the flex container. If the leftover
free-space is negative this value is identical to ‘flex-start’. Otherwise, the cross-start edge of the first line in the
flex container is placed flush with the cross-start content edge of the flex
container, the cross-end edge of the last
line in the flex container is placed flush with the cross-end content edge of the flex
container, and the remaining lines in the flex container are distributed
so that the empty space between any two adjacent lines is the same.
‘space-around’
Lines are evenly distributed in the flex container, with half-size
spaces on either end. If the leftover free-space is negative this value
is identical to ‘center’. Otherwise, the lines in the flex
container 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 flex container are half the size of the other
empty spaces.
‘stretch’
Lines stretch to take up the remaining space. If the leftover
free-space is negative, this value is identical to ‘flex-start’. Otherwise, the free-space is split
equally between all of the lines, increasing their cross size.
Note: Only flex containers with multiple lines ever have
free space in the cross-axis for lines to be aligned in, because in
a flex container with a single line the sole line automatically stretches
to fill the space.
An illustration of the ‘align-content’ keywords and their effects on
a multi-line flex container.
8.5. Flex Baselines
The baselines of a flex container are determined as follows (after
reordering with ‘order’):
Otherwise, if the flex container has at least one flex item, and its first flex item has a baseline parallel to the
flex container's main axis, the flex
container's main-axis baseline
is that baseline.
Otherwise, the flex container's main-axis baseline is synthesized
from the first item's content box, or, failing that, from the flex
container's content box.
cross-axis baseline
If the flex container has at least one flex
item, and its first flex item
has a baseline parallel to the flex container's cross axis, the flex container's cross-axis baseline is that
baseline.
Otherwise, the flex container's cross-axis baseline is synthesized
from the first item's content box, or, failing that, from the flex
container's content box.
When calculating the baseline according to the above rules, if the box
contributing a baseline has an ‘overflow’
value that allows scrolling, the box must be treated as being in its
initial scroll position for the purpose of determining its baseline.
CSS 2.1 did not define the baseline of block or table boxes. It is
expected that they will be defined consistent with those of table
cells, as follows:
‘block’
‘list-item’
The inline-axis baseline of a block is the baseline of the first
in-flow line box in the block, or the first in-flow block-level child
in the block that has a baseline, whichever comes first. If there is no
such line box or child, then the block has no baseline. For the
purposes of finding a baseline, in-flow boxes with a scrolling
mechanisms (see the ‘overflow’
property) must be considered as if scrolled to their origin position.
A block has no block-axis baseline.
‘table’
The inline-axis baseline of a table box is the baseline of its first
row. However, when calculating the baseline of an inline-block, table
boxes must be skipped.
The block-axis baseline of a table is undefined.
9. Flex Layout Algorithm
This section contains normative algorithms detailing the exact layout
behavior of a flex container and its contents. The algorithms here are
written 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 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.
For the purposes of these definitions, a definite size is one that can be
determined without measuring content, i.e. is a <length>, a
size of the initial containing block, or a <percentage> that
is resolved against a definite size. If a single-lineflex container has a definite cross
size, any flex items with ‘align-self: stretch’ must also be treated as having a
definite cross size. An indefinite size is one that is
not definite.
The following sections define the algorithm for laying out a flex
container and its contents.
9.1. Initial Setup
Generate anonymous flex items as
described in the Flex Items section.
Re-order the flex items and absolutely
positioned flex container children according to their ‘order’. The
elements with the lowest (most negative) ‘order’ values are first in the ordering. If
multiple elements share an ‘order’ value, they're ordered by document
order. This affects the order in which the elements generate boxes in the
box-tree, and how the rest of this algorithm deals with the generated
flex items.
9.2. Line Length Determination
Determine the available main and cross
space for the flex items. For each dimension, if that dimension
of the flex container is a definite
size, use that; otherwise, subtract the flex container's margin,
border, and padding from the space available to the flex container in
that dimension and use that value. This might result in
an infinite value.
Determine the flex
base size and hypothetical main
size of each item:
If the flex basis is ‘auto’ or depends
on its available size, and the flex container is being sized under a
min-content or max-content constraint (e.g. when performing automatic
table layout[CSS21]), size the item under that
constraint. The flex base size is
the item's resulting main size.
If flex item has an intrinsic aspect ratio, the flex item has
‘align-self: stretch’, the flex basis and
cross size are both ‘auto’ , and the flex
container is single-line and has a definite cross size, the flex base size is computed from the
flex container's inner cross size (clamped to the flex item’s min and max cross size) and the flex item’s intrinsic aspect ratio.
Otherwise, if the flex basis is ‘auto’
or depends on its available size, the available main size is infinite,
and the flex item's inline axis is parallel to the main axis, lay the
item out using the
rules for a box in an orthogonal flow[CSS3-WRITING-MODES].
The flex base size is the item's
max-content main size.
Otherwise, lay out the item into the available space using its flex
basis in place of its main size, and treating ‘auto’ as ‘max-content’.
The flex base size is the item's
resulting main size.
Determine the main size of the flex
container using the rules of the formatting context in which it
participates. For this computation, ‘auto’
margins on flex items are treated as ‘0’.
9.3. Main Size Determination
Collect flex items into flex
lines:
If the flex container is single-line, collect all the flex items
into a single flex line.
Otherwise, starting from the first uncollected item, collect
consecutive items one by one until the first time that the
next collected item would not fit into the flex container's
inner main size, or until a forced break is encountered. If the very
first uncollected item wouldn't fit, collect just it into the line. A
break is forced wherever the CSS2.1 ‘page-break-before/after’ [CSS21] or the CSS3 ‘break-before/after’ [CSS3-BREAK] properties
specify a fragmentation break.
Repeat until all flex items have been collected into flex lines.
Note that the "collect as many" line will collect
zero-sized flex items onto the end of the previous line even if the
last non-zero item exactly "filled up" the line.
Determine the hypothetical cross size of each
item by performing layout with the used main size and the
available space, treating ‘auto’ as ‘fit-content’.
Calculate the cross size of each flex
line.
If the flex container has only a single line (even if it's a multi-line flex container) and has a definite cross size, the cross size of
the flex line is the flex container's inner cross size.
Otherwise, for each flex line:
Collect all the flex items whose inline-axis is parallel to the
main-axis, whose ‘align-self’ is ‘baseline’, and
whose cross-axis margins are both non-‘auto’. Find the largest of the distances between
each item's baseline and its hypothetical outer cross-start edge, and
the largest of the distances between each item's baseline and its
hypothetical outer cross-end edge, and sum these two values.
Among all the items not collected by the previous step, find the
largest outer hypothetical cross
size.
The used cross-size of the flex line is the largest of the numbers
found in the previous two steps and zero.
Handle ‘align-content:
stretch’. If the flex container has a definite cross size, ‘align-content’ is
‘stretch’, and the sum of the flex lines' cross
sizes is less than the flex container's inner cross size, increase the
cross size of each flex line by equal amounts such that the sum of their
cross sizes exactly equals the flex container's inner cross size.
Collapse ‘visibility:collapse’ items. If any flex items
have ‘visibility: collapse’, note the cross
size of the line they're in as the item's strut
size, and restart layout from the beginning.
In this second layout round, when collecting items into lines, treat the
collapsed items as having zero main size. For the rest of the algorithm
following that step, ignore the collapsed items entirely (as if they
were ‘display:none’) except that after calculating the cross size of the lines, if
any line's cross size is less than the largest strut size among all the collapsed
items in the line, set its cross size to that strut size.
Skip this step in the second layout round.
Determine the used cross size of each flex
item. If a flex item has ‘align-self:
stretch’, its computed cross size property is ‘auto’, and neither of its cross-axis margins are
‘auto’, the used outer cross size is the used
cross size of its flex line, clamped according to the item's min and max
cross size properties. Otherwise, the used cross size is the item's hypothetical cross size.
Note that this step does not affect the main size of the
flex item, even if it has an intrinsic aspect ratio.
9.5. Main-Axis Alignment
Distribute any remaining free
space. For each flex line:
If the remaining free space is positive and at least one main-axis
margin on this line is ‘auto’, distribute
the free space equally among these margins. Otherwise, set all ‘auto’ margins to zero.
Resolve cross-axis ‘auto’ margins. If a flex item has ‘auto’ cross-axis margins:
If its outer cross size (treating those ‘auto’ margins as zero) is less than the cross size
of its flex line, distribute the difference in those sizes equally to
the ‘auto’ margins.
Otherwise, if the start or block-before margin
(whichever is in the cross axis) is ‘auto’,
set it to zero; set the opposite margin so that the outer cross size of
the item equals the cross size of its flex line.
Align all flex items along the
cross-axis per ‘align-self’, if neither of the item's
cross-axis margins are ‘auto’.
Determine the flex container's used
cross size:
If the cross size property is a definite size, use that.
Otherwise, use the sum of the flex lines' cross sizes.
To resolve the flexible lengths of the items within a flex line:
Determine the used flex factor. Sum the outer hypothetical main sizes of all
items on the line. If the sum is less than the flex container's inner
main size, use the flex grow
factor for the rest of this algorithm; otherwise, use the flex shrink factor.
Size inflexible items. For any items that have a
flex factor of zero, set their used main size to their hypothetical main size.
Check that you can distribute any space. If all the
flex items on the line are either frozen or have a flex factor of zero,
exit the algorithm.
Calculate free space. Sum the outer flex base sizes of all items on the
line, and subtract this from the flex container's inner main size. This
is the free space.
Distribute free space proportional to the flex
factors. If the sign of the free space is positive and the
algorithm is using the flex grow
factor, or if the sign of the free space is negative and the
algorithm is using the flex shrink
factor, distribute the free space to each flexible item's main
size in proportion to the item's flex factor:
If the free space is positive
Find the ratio of the item's flex
grow factor to the sum of the flex grow factors of all items on
the line. Set the item's main size to its flex base size plus a fraction of the
free space proportional to the ratio.
If the free space is negative
For every item on the line, multiply its flex shrink factor by its outer
flex base size, and note this as
its scaled flex shrink factor.
Find the ratio of the item's scaled flex shrink factor
to the sum of the scaled flex
shrink factors of all items on the line. Set the item's main
size to its flex base size minus a
fraction of the absolute value of the free space proportional to the
ratio. Note this may result in a negative inner main
size; it will be corrected in the next step.
Fix min/max violations. Clamp each item's main size
by its min and max main size properties. If the item's main size was made
smaller by this, it's a max violation. If the item's main size was made
larger by this, it's a min violation.
The total violation is the sum of the adjustments from the previous
step (clamped size - unclamped size).
If the total violation is:
Zero
Exit the algorithm.
Positive
Freeze all the items with min violations, reset all other items to
their size upon entering this algorithm, and return to step 2 of this
algorithm.
Negative
Freeze all the items with max violations, reset all other items to
their size upon entering this algorithm, and return to step 2 of this
algorithm.
9.8. Intrinsic Sizes
The max-content main size of a flex
container is the sum of the flex container's items' max-content
contributions. The min-content main size of a single-line flex container is the sum of
the flex container's items' min-content contributions; for a multi-line container, it is the largest of
those contributions.
The min-content cross size and max-content cross size of
a flex container are the cross size of the flex container after performing
layout into the given available main-axis space and infinite
available cross-axis space.
The main-size min-content/max-content
contribution of a flex item is its
outer hypothetical main size
when sized under a min-content/max-content
constraint (respectively).
See [CSS3-SIZING] for a definition
of the terms in this section.
10. Fragmenting Flex Layout
Flex containers can break across pages between items, between lines of
items (in multi-line mode), and inside
items. The ‘break-*’ properties apply to flex
containers as normal for block-level or inline-level boxes. This section
defines how they apply to flex items and elements inside flex items.
The following breaking rules refer to the fragmentation container as
the “page”. The same rules apply to any other fragmenters. (Substitute
“page” with the appropriate fragmenter type as needed.) See the CSS3
Fragmentation Module [CSS3-BREAK]. For readability,
in this section the terms "row" and "column" refer to the relative
orientation of the flex container with respect to the block flow direction
of the fragmentation context, rather than to that of the flex container
itself.
The exact layout of a fragmented flex container is not defined in this
level of Flexible Box Layout. However, breaks inside a flex container are
subject to the following rules:
In a row flex container, the ‘break-before’ and ‘break-after’ properties on flex items are
propagated to the flex line. The ‘break-before’ property on the first line and the
‘break-after’ property on the last line
are propagated to the flex container.
In a column flex container, the ‘break-before’ property on the first item and the
‘break-after’ property on the last item
are propagated to the flex container. Forced breaks on other items are
applied to the item itself.
A forced break inside a flex item effectively increases the size of
its contents; it does not trigger a forced break inside sibling items.
When a flex container is continued after a break, the space available
to its flex items (in the block flow
direction of the fragmentation context) is reduced by the space consumed
by flex container fragments on previous pages. The space consumed by a
flex container fragment is the size of its content box on that page. If
as a result of this adjustment the available space becomes negative, it
is set to zero.
If the first fragment of the flex container is not at the top of the
page, and some of its flex items don't fit in the remaining space on the
page, the entire fragment is moved to the next page.
When a multi-line colum flex
container breaks, each fragment has its own "stack" of flex lines, just
like each fragment of a multi-column element has its own row of column
boxes.
Aside from the rearrangement of items imposed by the previous point,
UAs should attempt to minimize distortation of the flex container with
respect to unfragmented flow.
10.1. Sample Flex
Fragmentation Algorithm
This informative section presents a possible fragmentation algorithm
for flex containers. Implementors are encouraged to improve on this
algorithm and provide feedback to the CSS Working
Group.
This algorithm assumes that pagination always proceeds only
in the forward direction; therefore, in the algorithms below, alignment
is mostly ignored prior to pagination. Advanced layout engines may be
able to honor alignment across fragments.
Lay out as many consecutive flex items or item fragments as
possible (but at least one or a fragment thereof), starting from the
first, until there is no more room on the page or a forced break is
encountered.
If the previous step ran out of room and the free space is
positive, the UA may reduce the distributed free space on this page
(down to, but not past, zero) in order to make room for the next
unbreakable flex item or fragment. Otherwise, the item or fragment
that does not fit is pushed to the next page. The UA should pull up if
more than 50% of the fragment would have fit in the remaining space
and should push otherwise.
If there are any flex items or fragments not laid out by the
previous steps, rerun the flex layout algorithm from Line Length Determination through Cross Sizing Determination with the next
page's size and all the contents (including those already
laid out), and return to the previous step, but starting from the
first item or fragment not already laid out.
For each fragment of the flex container, continue the flex layout
algorithm from Main-Axis Alignment to
its finish.
It is the intent of this algorithm that column-direction
single-line flex containers paginate
very similarly to block flow. As a test of the intent, a flex container
with ‘justify-content:start’ and no flexible
items should paginate identically to a block with in-flow children with
same content, same used size and same used margins.
Run the flex layout algorithm with regards to pagination
(limiting the flex container's maximum line length to the space left
on the page) through Cross Sizing
Determination.
Lay out as many flex lines as possible (but at least one) until
there is no more room in the flex container in the cross dimension or
a forced break is encountered:
Lay out as many consecutive flex items as possible (but at least
one), starting from the first, until there is no more room on the
page or a forced break is encountered. Forced breaks within
flex items are ignored.
If this is the first flex container fragment, this line contains
only a single flex item that is larger than the space left on the
page, and the flex container is not at the top of the page already,
move the flex container to the next page and restart flex container
layout entirely.
If there are any flex items not laid out by the first step,
rerun the flex layout algorithm from Main
Sizing Determination through Cross
Sizing Determination using only the items not laid out on a
previous line, and return to the previous step, starting from the
first item not already laid out.
If there are any flex items not laid out by the previous step,
rerun the flex layout algorithm from Line
Sizing Determination through Cross Sizing
Determination with the next page's size and only the items not
already laid out, and return to the previous step, but starting from
the first item not already laid out.
For each fragment of the flex container, continue the flex layout
algorithm from Main-Axis Alignment to
its finish.
If a flex item does not entirely fit on a single page, it
will not be paginated in multi-line column flex containers.
Run the entire flex layout algorithm (without regards to
pagination), except treat any ‘align-self’ other than ‘start’ or ‘baseline’ as ‘start’.
If an unbreakable item doesn't fit within the space left on the
page, and the flex container is not at the top of the page, move the
flex container to the next page and restart flex container layout
entirely.
For each item, lay out as much of its contents as will fit in the
space left on the page, and fragment the remaining content onto the
next page, rerunning the flex layout algorithm from Line Length Determination through Main-Axis Alignment into the new page size
using all the contents (including items completed on previous
pages).
Any flex items that fit entirely into previous
fragments still take up space in the main axis in later fragments.
For each fragment of the flex container, rerun the flex layout
algorithm from Cross-Axis Alignment to
its finish. For all fragments besides the first, treat ‘align-self’ and
‘align-content’ as being ‘start’ for all item fragments and lines.
If any item, when aligned according to its original ‘align-self’ value
into the combined cross size of all the flex container fragments,
would fit entirely within a single flex container fragment, it may be
shifted into that fragment and aligned appropriately.
Lay out as many flex lines as possible (but at least one),
starting from the first, until there is no more room on the page or a
forced break is encountered.
If a line doesn't fit on the page, and the line is not at the top
of the page, move the line to the next page and restart the flex
layout algorithm entirely, using only the items in and following this
line.
If a flex item itself causes a forced break, rerun the flex layout
algorithm from Main Sizing Determination
through Main-Axis Alignment, using only
the items on this and following lines, but with the item causing the
break automatically starting a new line in the line breaking step, then continue with
this step. Forced breaks within flex items are ignored.
If there are any flex items not laid out by the previous step,
rerun the flex layout algorithm from Line
Length Determination through Main-Axis
Alignment with the next page's size and only the items not already
laid out. Return to the previous step, but starting from the first
line not already laid out.
For each fragment of the flex container, continue the flex layout
algorithm from Cross Axis Alignment to
its finish.
11. Conformance
11.1. Document conventions
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.
11.2. Conformance
classes
Conformance to CSS Flexible Box Layout Module is defined for three
conformance classes:
A style sheet is conformant to CSS Flexible Box 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 Flexible Box Layout Module if, in
addition to interpreting the style sheet as defined by the appropriate
specifications, it supports all the features defined by CSS Flexible Box
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 Flexible Box 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.
11.3. Partial implementations
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.
11.4. Experimental
implementations
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.
11.5. Non-experimental
implementations
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.
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:
independent
each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
interoperable
passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent
tests must be made publicly available for the purposes of peer review.
implementation
a user agent which:
implements the specification.
is available to the general public. The implementation may be a
shipping product or other publicly available version (i.e., beta
version, preview release, or “nightly build”). Non-shipping product
releases must have implemented the feature(s) for a period of at least
one month in order to demonstrate stability.
is not experimental (i.e., a version specifically designed to pass
the test suite and is not intended for normal usage going forward).
The specification will remain Candidate Recommendation for at least six
months.
Acknowledgments
Thanks for feedback and contributions to Erik Anderson, Tony Chang, Phil
Cupp, Arron Eicholz, James Elmore, Andrew Fedoniouk, Brian Heuston,
Shinichiro Hamaji, Daniel Holbert, Ben Horst, John Jansen, Brad Kemper,
Kang-hao Lu, Markus Mielke, Robert O'Callahan, Christoph Päper, Ning
Rogers, Peter Salas, Morten Stenshorne, Christian Stockwell, Ojan Vafai,
Eugene Veselov, Boris Zbarsky.
Reverted the initial value of ‘min-width’/‘min-height’ back to being ‘0’ (not ‘auto’).
Specified that percentage margins/paddings on flex items are resolved
against their respective dimensions, not the inline dimension of the
containing block like blocks do.
Correct an omission in the flex-line size determination, so a single-line flexbox will size to its
contents if it doesn't have a definite size.
The following significant clarifications were also made:
Absolutely positioned children of a flex container are no longer
called "flex items" (to avoid terminology confusion).
Clarified that ‘float’ still makes
the children of a flex container turn into block-level elements.
Clarified that ‘overflow’ applies to
flex containers.
Clarified that ‘::first-line’ and ‘::first-letter’ pseudo-elements do not apply to flex
containers (because they are not block containers).
Clarify that ‘stretch’ checks for the computed value of
the cross-size property being ‘auto’, which
means that percentage cross-sizes that behave as ‘auto’ (because they don't resolve against definite
sizes) aren't stretched.
Clarified that ‘order’-modified document order is used
instead of raw document order when painting. (This was already stated in
the ‘order’ section,
but not in the section explicitly about painting order.)
