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<pre class='metadata'>
Title: CSS Positioned Layout Module Level 3
Status: ED
Work Status: Exploring
Shortname: css-position
Level: 3
Group: csswg
ED: https://drafts.csswg.org/css-position/
TR: https://www.w3.org/TR/css-position-3/
Previous Version: https://www.w3.org/TR/2016/WD-css-position-3-20160517/
Previous Version: https://www.w3.org/TR/2015/WD-css3-positioning-20150203/
Previous Version: https://www.w3.org/TR/2012/WD-css3-positioning-20120207/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com, w3cid 49885
Former Editor: Arron Eicholz, Microsoft, arronei@microsoft.com, w3cid 37934
!Issues list: <a href="http://wiki.csswg.org/spec/css-position/">in Wiki</a>
Abstract: This module contains defines coordinate-based positioning and offsetting schemes of <a href="https://www.w3.org/TR/CSS/">CSS</a>: [=relative positioning=], [=sticky positioning=], [=absolute positioning=], and [=fixed positioning=].
Link Defaults: css-transforms-1 (property) transform
</pre>
<pre class=link-defaults>
spec:css-writing-modes-4; type:dfn; text:end
spec:css2; type:value; for:float; text:none
spec:css-align-3; type:value; for:align-self; text:center
</pre>
<style type="text/css">
.switch > dt { font: inherit; }
.cb-example-table {
border: 1px solid;
border-collapse: collapse;
font-size: smaller;
text-align: center;
margin: 0 2em;
}
.cb-example-table td, .cb-example-table th {
border: 1px solid;
padding: 0 3px;
}
code span, td span {
white-space: nowrap;
}
.relationship-table {
border: 1px solid;
border-collapse: collapse;
font-size: smaller;
margin: 0 6em;
}
.relationship-table th {
text-align: center;
white-space:nowrap;
}
.relationship-table td, .relationship-table th {
border: 1px solid;
padding: 0 3px;
}
h2 dfn, h3 dfn {
font: inherit;
}
table.lpr {
border-collapse: collapse;
}
.lpr td, .lpr th {
border: 1px solid;
text-align: center;
}
.lpr col {
background: rgb(221, 238, 255);
border-right: 2px solid;
font-weight: bold;
padding: 0 5px;
}
.lpr div {
font-weight: bold;
-moz-transform: rotate(-90deg) translatex(-0.5em);
-ms-transform: rotate(-90deg) translatex(-0.5em);
-o-transform: rotate(-90deg) translatex(-0.5em);
-webkit-transform: rotate(-90deg) translatex(-0.5em);
transform: rotate(-90deg) translatex(-0.5em);
width: 1.5em;
}
.lpr th:first-child {
border: none;
background: white;
}
.lpr tbody td:first-child {
font-weight: normal;
}
.lpr thead td {
background: rgb(238, 238, 238);
font-weight: bold;
padding: 0 5px;
vertical-align: bottom;
white-space: nowrap;
}
.lpr thead tr:nth-child(2) {
border-bottom: 2px solid;
}
#abspos-auto ul { list-style-type: none; margin: 0; }
#abspos-auto caption { caption-side: bottom; }
</style>
<pre class=anchors>
urlPrefix: https://www.w3.org/TR/CSS2/visuren.html; spec: CSS2;
url: #normal-flow; text: normal flow; type: dfn;
url: #x1; text: viewport; type: dfn;
urlPrefix: https://www.w3.org/TR/CSS2/visudet.html; spec: CSS2;
url: #static-position; text: static position; type: dfn;
url: #static-position; text: static-position containing block; type: dfn;
</pre>
<pre class=link-defaults>
spec: css-align-3; type: value; text: stretch
</pre>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
The CSS layout algorithms, by default,
size and position boxes in relation to each other
so that nothing overlaps.
This specification defines several ways to violate these assumptions when needed,
moving elements around in ways that can make them overlap other content:
* [=Relative positioning=], which visually shifts a box relative to its laid-out location.
* [=Sticky positioning=], which visually shifts a box relative to its laid-out location
in order to keep it visible when a scrollable ancestor
would otherwise scroll it out of sight.
* [=Absolute positioning=], which ignores normal layout entirely,
pulling the element [=out of flow=] and
positioning it relative to its [=containing block=]
with no regard for other content.
* [=Fixed positioning=], which absolutely positions the box
and affixes it to the viewport or page frame
so that it is always visible.
These [=positioning schemes=],
controlled by the 'position' property and the [=inset properties=],
are powerful
but easy to misuse.
With appropriate care,
they allow many interesting and useful layouts
that couldn't otherwise be achieved with standard layout rules;
without, they allow a page to be laid out in an unusable overlapping jumble of content.
<h3 id="placement">
Module Interactions</h3>
This module replaces and extends the [=positioning scheme=] features
defined in [[!CSS2]] sections:
<ul>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#containing-block">9.1.2 Containing blocks</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#positioning-scheme">9.3 Positioning schemes</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#relative-positioning">9.4.3 Relative positioning</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#absolute-positioning">9.6 Absolute positioning</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">9.7 Relationships between display, position, and float</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#comparison">9.8 Comparison of normal flow, floats, and absolute positioning</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#containing-block-details">10.1 Definition of "containing block"</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">10.3.7 Absolutely positioned, non-replaced elements</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-replaced-width">10.3.8 Absolutely positioned, replaced elements</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">10.6.4 Absolutely positioned, non-replaced elements</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-replaced-height">10.6.5 Absolutely positioned replaced elements</a>
</ul>
It also replaces and supersedes the 'inset*' property definitions in [[CSS-LOGICAL-1]]
([[CSS-LOGICAL-1#inset-properties]]).
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<h2 id="position-property">
Choosing A Positioning Scheme: 'position' property</h2>
<pre class="propdef">
Name: position
Value: static | relative | absolute | sticky | fixed
Initial: static
Applies to: all elements except table-column-group and table-column
Inherited: no
Animatable: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
The 'position' property determines which of the <dfn export lt="positioning scheme">positioning schemes</dfn>
is used to calculate the position of a box.
Values other than ''static'' make the box a <dfn export lt="positioned box|positioned">positioned box</dfn>,
and cause it to establish an [=absolute positioning containing block=] for its descendants.
Values have the following meanings:
<dl dfn-for="position" dfn-type="value">
<dt><dfn>static</dfn>
<dd>
The box is not a [=positioned box=],
and is laid out according to the rules of its parent [=formatting context=].
The [=inset properties=] do not apply.
<dt><dfn>relative</dfn>
<dd>
The box is laid out as for ''static'',
then offset from the resulting position.
This offsetting is a purely visual effect,
and does not affect the size or position of any other box,
except insofar as it increases the [=scrollable overflow area=] of its ancestors.
This [=positioning scheme=] is called
<dfn export dfn for="" lt="relative position | relatively position | relatively-positioned | relatively positioned box" local-lt="relative|relatively">relative positioning</dfn>.
<dt><dfn>sticky</dfn>
<dd>
Identical to ''relative'',
except that its offsets are automatically adjusted
in reference to the nearest ancestor [=scroll container's=] [=scrollport=]
(as modified by the [=inset properties=])
in whichever axes the [=inset properties=] are not both ''top/auto'',
to try to keep the box in view within its [=containing block=]
as the user scrolls.
This [=positioning scheme=] is called
<dfn export dfn for="" lt="sticky position | sticky-positioned | sticky-positioned box" local-lt="sticky">sticky positioning</dfn>.
<dt><dfn>absolute</dfn>
<dd>
The box is taken [=out of flow=]
such that it has no impact on the size or position of its siblings and ancestors,
and does not participate in its parent’s [=formatting context=].
Instead, the box is positioned and sized solely in reference to
its [=absolute positioning containing block=],
as modified by the box's [=inset properties=],
see [[#abspos-layout]].
It can overlap [=in-flow=] content
or other [=absolutely positioned=] elements,
and is included in the [=scrollable overflow area=]
of the box that generates is [=containing block=].
This [=positioning scheme=] is called
<dfn export dfn for="" lt="absolute position | absolutely position | absolutely-positioned | absolutely positioned box | absolutely positioned element" local-lt="absolute|absolutely">absolute positioning</dfn>.
<dt><dfn>fixed</dfn>
<dd>
Same as ''absolute'',
except the box is positioned and sized relative to a [=fixed positioning containing block=]
(usually the [=viewport=] in [=continuous media=], or the [=page area=] in [=paged media=]).
The box’s position is fixed with respect to this reference rectangle:
when attached to the [=viewport=]
it does not move when the document is scrolled,
and when attached to the [=page area=]
is replicated on every page when the document is paginated.
This [=positioning scheme=] is called
<dfn export dfn for="" lt="fixed position | fixed-positioned | fixed-positioned box" local-lt="fixed">fixed positioning</dfn>
and is considered a subset of [=absolute positioning=].
<div class="example">
Authors may wish to specify ''fixed'' in a media-dependent way.
For instance, an author may want a box to remain at the top of the <a>viewport</a> on the screen,
but not at the top of each printed page.
The two specifications may be separated by using an <a href="https://www.w3.org/TR/CSS2/media.html#at-media-rule">'@media'</a> rule, as in:
<pre highlight=css>
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
</pre>
</div>
</dl>
A 'position' value of ''position/absolute'' or ''position/fixed''
[=blockifies=] the box
and causes 'float' to compute to ''float/none''.
<h3 id="def-cb">
Containing Blocks of Positioned Boxes</h3>
The [=containing block=] of a ''position/static'', ''position/relative'', or ''position/sticky'' [=box=]
is as defined by its [=formatting context=].
For ''position/fixed'' and ''position/absolute'' boxes,
it is defined as follows:
<dl class=switch>
<dt id="absolute-cb">If the box has ''position: absolute'':
<dd>
The [=containing block=] is established
by the nearest ancestor box that establishes an <dfn export>absolute positioning containing block</dfn>,
in the following way:
<dl class=switch>
<dt>If the ancestor is not an [=inline box=],
<dd>
the [=containing block=] is formed by the [=padding edge=] of the ancestor,
unless otherwise specified
(for example, see [[CSS-GRID-1#abspos-items]]).
<dt>If the ancestor is an [=inline box=],
<dd>
the [=containing block=] is formed by
the [=block-start=] and [=inline-start=] [=content edges=] of the first [=box fragment=] of the ancestor,
and the [=block-end=] and [=inline-end=] [=content edges=] of the last [=box fragment=] of the ancestor.
Note: If the ancestor breaks across a line,
the “start” position might more end-ward than the “end” position.
</dl>
If no ancestor establishes one,
the [=absolute positioning containing block=] is
the [=initial containing block=].
Note: Properties that can cause a box to establish
an [=absolute positioning containing block=] include
'position', 'transform', 'will-change', 'contain'…
<dt id="fixed-cb">If the box has ''position: fixed'':
<dd>
The [=containing block=] is established
by the nearest ancestor box that establishes an <dfn export>fixed positioning containing block</dfn>,
with the bounds of the [=containing block=]
determined identically to the [=absolute positioning containing block=].
Note: Properties that can cause a box to establish
a [=fixed positioning containing block=] include
'transform', 'will-change', 'contain'…
If no ancestor establishes one,
the [=fixed positioning containing block=] is:
* in [=continuous media=],
the <a>viewport</a>;
as a result, [=fixed=] boxes do not move when the document is scrolled.
Note: In this respect, they are similar to
[[css-backgrounds-3#background-attachment|fixed background images]]
(''background-attachment: fixed'').
* in [=paged media=],
the [=page area=] of each page;
[=fixed positioned=] [=boxes=] are thus replicated on every page.
(They are fixed with respect to the page box only,
and are not affected by being seen through a [=viewport=];
as in the case of print preview, for example.)
Note: As a result, parts of [=fixed-positioned boxes=]
that extend outside the [=initial containing block=]/[=page area=]
cannot be scrolled to
and will not print.
</dl>
<div class="example">
With no positioning, the containing blocks (C.B.) in the following document:
<pre highlight=html>
<!DOCTYPE html>
<html>
<head>
<title>Illustration of containing blocks</title>
</head>
<body id="body">
<div id="div1">
<p id="p1">This is text in the first paragraph...</p>
<p id="p2">This is text <em id="em1"> in the
<strong id="strong1">second</strong> paragraph.</em></p>
</div>
</body>
</html>
</pre>
are established as follows:
<table class="cb-example-table">
<tr><th>For box generated by <th>C.B. is established by
<tr><td>html<td>initial C.B. (UA-dependent)
<tr><td>body<td>html
<tr><td>div1<td>body
<tr><td>p1<td>div1
<tr><td>p2<td>div1
<tr><td>em1<td>p2
<tr><td>strong1<td>p2
</table>
If we position "div1":
<pre highlight=css>
#div1 { position: absolute; left: 50px; top: 50px }
</pre>
its containing block is no longer "body"; it becomes the <span>initial containing block</span>
(since there are no other positioned ancestor boxes).
If we position "em1" as well:
<pre highlight=css>
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
</pre>
the table of containing blocks becomes:
<table class="cb-example-table">
<tr><th>For box generated by <th>C.B. is established by
<tr><td>html<td>initial C.B. (UA-dependent)
<tr><td>body<td>html
<tr><td>div1<td>initial C.B.
<tr><td>p1<td>div1
<tr><td>p2<td>div1
<tr><td>em1<td>div1
<tr><td>strong1<td>em1
</table>
By positioning "em1", its containing block becomes the nearest positioned ancestor box
(i.e., that generated by "div1").
</div>
<h2 id="coords">
Positioning Coordinates</h2>
The precise location of a [=positioned box=] is controlled by
the <dfn>inset properties</dfn>:
the [=physical=] [=inset properties=] 'top', 'right', 'bottom', 'left';
the [=flow-relative=] [=inset properties=] 'inset-block-start', 'inset-inline-start', 'inset-block-end', and 'inset-inline-end';
and their [=shorthands=], 'inset-block', 'inset-inline', and 'inset'.
The interpretation of these [=inset properties=] varies by [=positioning scheme=]:
* for [=absolute positioning=], they represent
insets from the containing block.
* for [=relative positioning=], they represent
insets from the box’s original margin edge.
* for [=sticky positioning=], they represent
insets from the [=scrollport=] edge.
<h3 id="insets">
Box Insets: the 'top', 'right', 'bottom', 'left', 'inset-block-start', 'inset-inline-start', 'inset-block-end', and 'inset-inline-end' properties </h3>
<pre class="propdef">
Name: top, right, bottom, left, inset-block-start, inset-inline-start, inset-block-end, inset-inline-end
Value: auto | <<length-percentage>>
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to size of <a>containing block</a>; see prose
Computed value: the keyword ''top/auto'' or a computed <<length-percentage>> value
Animation type: by computed value type
</pre>
These [=inset properties=] represent an inward “inset”
on the corresponding side of the box
(with respect to the box’s own [=writing mode=];
see [[CSS-WRITING-MODES-3#abstract-box]]).
For example, 'top' represents a downward inset of the top edge.
The [=physical=] and [=flow-relative=] properties
interact as defined in [[!CSS-LOGICAL-1]].
Values have the following meanings:
<dl dfn-for="top,right,bottom,left, inset-block-start,inset-inline-start,inset-block-end,inset-inline-end,inset-block,inset-inline,inset" dfn-type="value">
<dt><dfn><length></dfn>
<dd>
The inset is a fixed distance from the reference edge.
Negative values are allowed.
<dt><dfn><percentage></dfn>
<dd>
The inset is a percentage
relative to the <a>containing block</a>’s size
in the corresponding axis
(e.g. width for 'left' or 'right', height for 'top' and 'bottom').
For [=sticky positioned=] boxes,
the inset is instead
relative to the relevant [=scrollport=]’s size.
Negative values are allowed.
<dt><dfn>auto</dfn></dt>
<dd>
Represents an unconstrained inset;
the exact meaning depends on the [=positioning scheme=].
</dl>
Note: For [=fixed positioned=] elements,
using large values or negative values
can easily move elements outside the <a>viewport</a>
and make the contents unreachable through scrolling or other means.
<h3 id="inset-shorthands">
Box Insets Shorthands: the 'inset-block', 'inset-inline', and 'inset' properties</h3>
<pre class="propdef">
Name: inset-block, inset-inline
Value: <'top'>{1,2}
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: by computed value type
</pre>
The 'inset-block' and 'inset-inline' properties are [=shorthand properties=]
for setting 'inset-block-start' + 'inset-block-end'
or 'inset-inline-start' + 'inset-inline-end',
respectively,
in a single declaration.
The first component value sets the [=CSS/start=] side,
the second sets the [=CSS/end=];
if omitted, the second value defaults to the first.
<pre class="propdef">
Name: inset
Value: <'top'>{1,4}
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: by computed value type
</pre>
The 'inset' property is a [=shorthand property=]
that sets all of the [=inset properties=]
in a single declaration,
assigning values to the longhands representing each side
exactly as the 'margin' property does for its longhands.
<div class=note>
By default, the 'inset' property values
are assigned to the corresponding <em>[=physical=]</em> [=longhand properties=]--
'top', 'right', 'bottom', and 'left'--
which for historical reasons do not have an <css>inset-</css> prefix.
This matches the behavior of other "4 values assigned to sides" properties,
such as 'margin'.
Allowing properties such as this to resolve to the [=flow-relative=] [=longhands=] is under discussion in [[CSS-LOGICAL-1]].
Yes, we understand it's a little confusing
that 'inset' doesn't expand to any 'inset-*' properties.
</div>
<h3 id="relpos-insets" oldids="relpos">
Relative Positioning</h3>
For a [=relatively positioned=] box,
the [=inset properties=] move the box inward
from the respective edge,
without changing its size.
'left' moves the box to the right, 'right' moves it to the left, etc.
Since boxes are not split or stretched as a result of [=relative positioning=]
opposing [=used values=] in a given axis must be negations of each other:
<ul>
<li>
If opposing [=inset properties=] in an axis both compute to ''top/auto''
(their [=initial values=]),
their [=used values=] are zero
(i.e., the boxes stay in their original position in that axis).
<li>
If only one is ''top/auto'',
its [=used value=] becomes the negation of the other,
and the box is shifted by the specified amount.
<li>
If neither is ''top/auto'',
the position is over-constrained;
(with respect to the [=writing mode=] of its [=containing block=])
the [=computed value|computed=] [=CSS/end=] side value is ignored,
and its [=used value=] becomes the negation of the [=CSS/start=] side.
</ul>
<div class="example">
The following three rules are equivalent,
and shift the box ''1em'' to the left:
<pre highlight=css>
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
</pre>
</div>
If specified on
a ''table-row-group'', ''table-header-group'', ''table-footer-group'', or ''table-row'' [=box=]
the shift affects all the contents of the box,
including all <a value lt="table-cell">table cells</a> that originate in the affected row,
but not those that don't.
Note: Since 'position' does not apply to ''table-column-group'' or ''table-column'' boxes,
they are not affected by [=relative positioning=].
<h3 id="stickypos-insets" oldids="sticky-pos">
Sticky positioning</h3>
[=Sticky positioning=] is similar to [=relative positioning=]
except the offsets are automatically calculated
in reference to the [=nearest scrollport=].
For a [=sticky positioned=] [=box=],
the [=inset properties=]
represent insets from the respective edges of the [=nearest scrollport=],
defining the <dfn export>sticky view rectangle</dfn>
used to constrain the box’s position.
(For this purpose an ''top/auto'' value represents a zero inset.)
If this results in a [=sticky view rectangle=] size in any axis
less than the size of the [=border box=] of the [=sticky=] box in that axis,
then the effective [=end=]-edge inset in the affected axis is reduced
(possibly becoming negative)
to bring the [=sticky view rectangle=]’s size up to
the size of the [=border box=] in that axis
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
<div class=example>
For example,
if the [=nearest scrollport=] is ''300px'' tall,
the [=sticky=] box's [=border box=] is ''200px'' tall,
and it has ''top: 20px'',
then the top-edge inset of the [=nearest scrollport=] is ''20px'',
and the bottom-edge inset is ''0px'',
yielding a [=sticky view rectangle=] that is ''280px'' tall.
But if the [=nearest scrollport=] were only ''100px'' tall,
then the effective bottom-edge inset becomes ''-120px'',
resulting in a [=sticky view rectangle=] that’s ''200px'' tall,
enough to fully contain the [=margin box=] of the [=sticky=] box.
</div>
For each side of the box,
if the corresponding [=inset property=] is not ''top/auto'',
and the corresponding [=border edge=] of the box would be outside
the corresponding edge of the [=sticky view rectangle=],
then the box must be visually shifted
([[#relpos-insets|as for relative positioning]])
to be inward of that [=sticky view rectangle=] edge,
insofar as it can while its [=position box=]
remains contained within its [=containing block=].
The <dfn>position box</dfn> is its [=margin box=],
except that for any side for which the distance between its [=margin edge=]
and the corresponding edge of its [=containing block=]
is less than its corresponding [=margin=],
that distance is used in place of that [=margin=].
Note: A sticky positioned element with
a non-''auto'' 'top' value and an ''auto'' 'bottom' value
will only ever be pushed down by sticky positioning;
it will never be offset upwards.
Note: Multiple [=sticky positioned=] boxes in the same container
are offset independently,
and therefore might overlap.
<h4 id="stickypos-scroll">
Scroll Position of Sticky-Positioned Boxes</h4>
For the purposes of any operation targeting the scroll position
of a sticky positioned element (or one of its descendants),
the sticky positioned element must be considered to be positioned
at its initial (non-offsetted) position.
<div class="example">
For example, if a user clicks a link targeting a [=sticky-positioned=] element,
even if the element's [=nearest scrollport=] is currently scrolled
such that the [=sticky positioned=] element is offset from its initial position,
the [=scrollport=] will be scrolled back
so that the element's initial position is visible.
</div>
<h3 id="abspos-insets" oldids="abs-pos,fixed-pos">
Absolute (and Fixed) Positioning</h3>
For an [=absolutely positioned=] box,
the [=inset properties=] effectively reduce the [=containing block=]
into which it is sized and positioned
by the specified amounts.
If only one [=inset property=] in a given axis is ''top/auto'',
it is set to zero.
If both [=inset properties=] in a given axis are ''top/auto'', then,
depending on the box’s [=self-alignment property=] in the relevant axis
(treating ''align-self/normal'' as ''align-self/start''
and any [=distributed alignment|distributed=], [=baseline alignment|baseline=], or ''align-self/stretch'' alignment value as its fallback alignment):
<dl class=switch>
<dt>for ''align-self/self-start'' alignment or its equivalent
<dd>
Set its start-edge [=inset property=] to the static position,
and its end-edge [=inset property=] to zero.
<dt>for ''align-self/self-end'' alignment or its equivalent
<dd>
Set its end-edge [=inset property=] to the static position,
and its start-edge [=inset property=] to zero.
<dt>for ''align-self/center'' alignment
<dd>
Let <var>start distance</var> be the distance from
the center of its [=static position rectangle=]
to the start edge of its [=containing block=],
and <var>end distance</var> be the distance from
the center of its [=static position rectangle=]
to the end edge of its [=containing block=].
If <var>start distance</var> is less than or equal to <var>end distance</var>,
then set the start-edge [=inset property=] to zero,
and set the end-edge [=inset property=] to
(<var>containing block size</var> - 2 × |<var>start distance</var>|);
otherwise,
set the end-edge [=inset property=] to zero
and the start-edge [=inset property=] to
(<var>containing block size</var> - 2 × |<var>end distance</var>|).
</dl>
If these adjustments result in
an effective [=containing block=] size in any axis less than zero,
then the [=weaker inset=] in the affected axis
is reduced (possibly becoming negative)
to bring that size up to zero.
In the case that only one inset is ''auto'',
that is the <dfn>weaker inset</dfn>;
otherwise the [=weaker inset=] is the inset of the [=end=] edge
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
Note: Sizing and positioning of the [=absolutely positioned box=]
into this <dfn>inset-modified containing block</dfn>
is as described in [[#abspos-layout]].
If its [=self-alignment property=] in an axis is ''align-self/normal'',
then the [=resolved value=] of its [=weaker inset=] in that axis
is the value necessary to match that edge of its [=inset-modified containing block=]
to the corresponding edge of its [=margin box=]
after [[#abspos-layout|layout]].
(Otherwise the [=resolved value=] is the [=used value=] described above.)
<h4 id="staticpos-rect">
Resolving Automatic Insets</h4>
When both [=inset properties=] in a given axis are ''left/auto'',
they are resolved in reference to the box’s <dfn export>static position rectangle</dfn>,
whose edges represent the [=static position=] of the box
from each side of its [=containing block=].
This [=static position rectangle=]
and the [=static positions=] to which it corresponds
are defined by the layout model
the box would have had if its 'position' were ''position/static''
(and its 'float' and 'clear' values were ''float/none'').
(Note that this may require assuming a different hypothetical value
for 'display' as well.)
<dl>
<dt>Block Layout
<dd>
The [=static positions=] of a [=block-level box=]
are defined in [[CSS2]] Chapter 10.
The [=static position rectangle=] is a zero-thickness rectangle spanning between
the inline-axis sides of the box’s [=static-position containing block=]
(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2§10.3.7</a>);
and positioned at its [=block-start=] [=static position=]
(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">CSS2§10.6.4</a>).
<dt>Inline Layout
<dd>
The [=static positions=] of an [=inline-level box=]
are defined in [[CSS2]] Chapter 10.
The [=static position rectangle=] is a zero-thickness rectangle spanning between
the [=line-over=]/[=line-under=] sides of the [=line box=]
that would have contained its “hypothetical box”
(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2§10.3.7</a>);
and positioned at its [=inline-start=] [=static position=].
<dt>Flex Layout
<dd>
The [=static position rectangle=] of the child of a [=flex container=]
corresponds to the [=content edges=] of the [=flex container=].
See <a href="https://www.w3.org/TR/css-flexbox-1/#abspos-items">static position of a grid container child</a> in [[!CSS-FLEXBOX-1]].
<dt>Grid Layout
<dd>
By default, the [=static position rectangle=] of the child of a [=grid container=]
corresponds to the [=content edges=] of the [=grid container=].
However, if that [=grid container=] also establishes
the box’s actual [=containing block=],
then the [=grid area=] specified by the [=grid-placement properties=]
establishes its [=static position rectangle=] instead.
See the <a href="https://www.w3.org/TR/css-grid-1/#static-position">static position of a grid container child</a> in [[!CSS-GRID-1]].
</dl>
For the purposes of calculating the [=static position rectangle=],
the [=containing block=] of [=fixed positioned=] elements
is the [=initial containing block=] instead of the [=viewport=],
and all [=scroll containers=] should be assumed
to be scrolled to their [=initial scroll position=].
Additionally, all ''margin/auto'' margins on the box itself
are treated as zero.
<h4 id="abspos-breaking" oldids="breaking">
Fragmenting Absolutely-positioned Elements</h4>
In a [=fragmented flow=], an [=absolutely positioned box=]
is positioned relative to its [=containing block=]
ignoring any [=fragmentation breaks=]
(as if the flow were continuous).
The box may subsequently be broken over several [=fragmentation containers=].
For absolutely positioned content in [=paged media=]
that resolves to a position on a page
other than the page being laid out (the current page),
or resolves to a position on the current page
that has already been rendered for printing,
printers may place the content:
<ul>
<li>on the current page,
<li>on a subsequent page, or
<li>may omit it altogether.
</ul>
Note: A [=block-level=] element that is split over several pages
can have a different width on each page,
and there may be device-specific limits.
User agents must not paginate the content of [=fixed-positioned boxes=].
Note: User agents might print invisible content in other ways.
See [[CSS-PAGE-3#content-outside-box]].
<h2 id="abspos-layout">
Absolute Positioning Layout Model</h2>
[=Absolute positioning=] not only takes a box [=out of flow=],
but also lays it out in its [=containing block=]
(after the final size of the [=containing block=] has been determined)
according to the <dfn export lt="absolute positioning layout | absolute positioning layout model">absolute positioning layout model</dfn>:
<ol>
<li>
First, its [=inset-modified containing block=] is calculated,
defining its [=available space=].
(See [[#abspos-insets]].)
Because an [=absolutely positioned box=] can have no effect on the size of its [=containing block=],
its [=available space=] is always [=definite=].
<li>
Next, its width and height are resolved against this [=definite=] [=available space=],
as its [=preferred size=] capped by its [=maximum size=] (if any), floored by its [=minimum size=].
See [[#abspos-auto-size]].
Percentages, however,
are resolved against the original [=containing block=] size.
<li>
Then, the value of any ''margin/auto'' margins are calculated,
see [[#abspos-margins]].
<li>
Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=]
as defined by its [=self-alignment properties=].
</ol>
<h3 id="abspos-auto-size">
Automatic Sizes of Absolutely-Positioned Boxes</h3>
The [=automatic size=] of an [=absolutely positioned box=] is resolved against its [=inset-modified containing block=] as follows
(treating ''margin/auto'' [=margins=] as zero):
<dl class=switch>
: If its [=self-alignment property=] in the relevant axis is ''align-self/stretch''
: Or if it is ''align-self/normal''
and the box is [=non-replaced=],
not a [=table wrapper box=],
and has no ''inset/auto'' [=inset property|inset=] in the relevant axis
::
Its [=automatic size=] is its [=stretch-fit size=].
: Otherwise
::
Its [=automatic size=] is its [=fit-content size=].
</dl>
However, if the box has an aspect-ratio,
then an [=automatic size=] in the [=ratio-dependent axis=]
is instead resolved as a [=max-content size=].
When both axes have an [=automatic size=],
if only one axis has an ''inset/auto'' [=inset property|inset=]
then that axis is the [=ratio-dependent axis=],
else the [=block axis=] is the [=ratio-dependent axis=].
An [=automatic size=] in the [=ratio-determining axis=]
is determined as above.
The [=automatic minimum size=] of an absolutely-positioned box is always zero.
<h3 id="abspos-margins">
Auto Margins of Absolutely-Positioned Boxes</h3>
If either [=inset property=] in the relevant axis is ''top/auto'',
then any ''margin/auto'' [=margins=] resolve to zero.
Otherwise, the <var>remaining space</var> is calculated
as the size of its [=inset-modified containing block=] in the relevant axis
minus the box's used size in the relevant axis,
and this <var>remaining space</var> is divided among
any ''margin/auto'' margins in the relevant axis.
However,
(all with respect to the [=writing mode=] of the [=containing block=]),
if in the [=inline axis=]
the <var>remaining space</var> is negative
and both margins are ''margin/auto'',
the [=inline-start|start=] margin resolves to zero
and the [=inline-end|end=] margin receives the <var>remaining space</var>.
<h2 id="abspos-old">
Old Absolute Positioning Layout Model</h2>
ISSUE: This section is being replaced with the new [[#abspos-layout]] section.
It is preserved here for comparison:
both models should yield the same result
in [=horizontal writing modes=] when the box’s [=self-alignment=] is ''align-self/normal''.
<h3 id="abs-non-replaced-width">
The Width of Absolutely-Positioned, Non-Replaced Elements</h3>
The constraint that determines the used values for these elements is:
<code>
'left' + 'margin-left' + 'border-left-width' + 'padding-left' + 'width' +
'padding-right' + 'border-right-width' + 'margin-right' +
'right' = width of containing block
</code>
If all three of 'left', 'width', and 'right' are ''left/auto'':
First set any ''margin/auto'' values for 'margin-left' and 'margin-right'
to ''0''.
Then, if the 'direction' property of
the element establishing the static-position <a>containing block</a>
is ''ltr'' set 'left' to the static position
and apply rule number <em>three</em> below;
otherwise, set 'right' to the static-position
and apply rule number <em>one</em> below.
If none of the three is ''left/auto'':
If both 'margin-left' and 'margin-right' are ''margin/auto'',
solve the equation under the extra constraint that
the two margins get equal values,
unless this would make them negative,
in which case when direction of the containing block is ''ltr'' (''rtl''),
set 'margin-left' ('margin-right') to ''0''
and solve for 'margin-right' ('margin-left').
If one of 'margin-left' or 'margin-right' is ''margin/auto'',
solve the equation for that value.
If the values are over-constrained,
ignore the value for 'left'
(in case the 'direction' property of the <a>containing block</a> is ''rtl'')
or 'right'
(in case 'direction' is ''ltr'') and solve for that value.
Otherwise,
set ''margin/auto'' values for 'margin-left' and 'margin-right' to ''0'',
and pick one of the following six rules that apply.
<ol>
<li>
If 'left' and 'width' are ''left/auto'' and 'right' is not ''right/auto'',
then the width is shrink-to-fit.
Then solve for 'left'.
<li>
If 'left' and 'right' are ''left/auto'' and 'width' is not ''width/auto'',
then if the 'direction' property of
the element establishing the static-position <a>containing block</a>