csswg-drafts/css-position/Overview.bs at efa08b6b6af441714783582a393518ff4c62576f · w3c/csswg-drafts · GitHub

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<h1>CSS Positioned Layout Module Level 3</h1>

<pre class='metadata'>
Status: ED
Work Status: Exploring
Shortname: css-position
Level: 1
Group: csswg
ED: http://dev.w3.org/csswg/css-position/
TR: http://www.w3.org/TR/css3-positioning/
Previous Version: http://www.w3.org/TR/2012/WD-css3-positioning-20120207/
Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com
Editor: Arron Eicholz, Microsoft, arronei@microsoft.com
!Issues list: <a href="http://wiki.csswg.org/spec/css-position/">in Wiki</a>
Abstract:This module contains the features of CSS level&nbsp;3 relating to positioning and stacking of elements. It includes and extends the functionality of CSS level&nbsp;2 ([[!CSS21]]), which builds on CSS level&nbsp;1 [[CSS1]]. The main extensions compared to level&nbsp;2 are the ability to position elements based on CSS Region boxes, the ability to specify different containing blocks for elements and sticky positioning. <p>Other kinds of layout, such as tables, "floating" boxes, ruby annotations, grid layouts, columns and basic handling of normal "flow" content, are described in other modules. Also, the layout of text inside each line is defined elsewhere.
Ignored Terms: div, dl, dfn, attachment-type, attachment-top, attachment-right, attachment-bottom, attachment-left, attachment
Link Defaults: css-transforms-1 (property) transform
</pre>

<style type="text/css">
  .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;
  }
</style>

<h2 id="intro">
Introduction</h2>
  <em>This section is not normative.</em>

  CSS assumes the document layout is modeled as a tree of elements. The unique
  element that has no parent is called the root element. This module describes
  how any of the elements from the tree of elements can be arranged independent
  of document order (i.e. taken out of "flow"). With a positioned element the
  element may be placed anywhere within the content not specifically respecting
  the tree of elements order.


  In [[!CSS21]], the visual formatting model explained how each element in the
  document tree generates zero or more boxes according to the box model. This
  module further explains and extends the positioning scheme. The layout of
  these boxes is governed by:

  <ul>
    <li>
      <a href="http://www.w3.org/TR/CSS2/box.html#box-dimensions">box dimensions</a>
      and <a href="http://www.w3.org/TR/CSS2/visuren.html#box-gen">type</a>.
    <li>
      positioning scheme (<a>normal flow</a>, float, and absolute positioning).
    <li>
      relationships between elements in the
      <a href="http://www.w3.org/TR/CSS2/conform.html#doctree">document tree</a>.
    <li>
      external information (e.g., viewport size, intrinsic dimensions of images, etc.).
  </ul>

  The properties defined in this module apply to both continuous media and paged
  media.

<h3 id="placement">
Module Interactions</h3>

  This module replaced and extends the positioning scheme features defined in
  [[!CSS21]] sections:

  <ul>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#viewport">9.1.1 The viewport</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#containing-block">9.1.2 Containing blocks</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#positioning-scheme">9.3 Positioning schemes</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#relative-positioning">9.4.3 Relative positioning</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#absolute-positioning">9.6 Absolute positioning</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">9.7 Relationships between display, position, and float</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#comparison">9.8 Comparison of normal flow, floats, and absolute positioning</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visuren.html#layers">9.9 Layered presentation</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visudet.html#containing-block-details">10.1 Definition of "containing block"</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">10.3.7 Absolutely positioned, non-replaced elements</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-width">10.3.8 Absolutely positioned, replaced elements</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">10.6.4 Absolutely positioned, non-replaced elements</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-height">10.6.5 Absolutely positioned replaced elements</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/visudet.html#root-height">10.6.7 Auto heights for block formatting context roots</a>
    <li>
    <a href="http://www.w3.org/TR/CSS2/zindex.html">Appendix E. Elaborate description of Stacking Contexts</a>
  </ul>

<!-- End section" Module Interactions -->

<h3 id="values">
Values</h3>

  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 &lt;length&gt; 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
  <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
  keyword as their property value. For readability it has not been repeated
 
F328
 explicitly.

<!-- End section: Values -->
<!-- End section Introduction -->

<h2 id="vp">
The <dfn lt="viewport">Viewport</dfn></h2>

    User agents for <a href="http://www.w3.org/TR/CSS2/media.html#continuous-media-group">continuous media</a> generally offer users a viewport (a window or other
    viewing area on the screen) through which users consult a document. User
    agents may change the document&#39;s layout when the viewport is resized
    (see the <span>initial containing block</span>).

    When the viewport is smaller than the area of the canvas on which the document
    is rendered, the user agent may offer a scrolling mechanism. There is at most
    one viewport per <a href="http://www.w3.org/TR/CSS2/intro.html#canvas">canvas</a>,
    but user agents may render to more than one canvas (i.e., provide different
    views of the same document).

<!-- End section: The Viewport -->

<h2 id="cb">
Containing Blocks</h2>

  In CSS, many box positions and sizes are calculated with respect to the edges
  of a rectangular box called a containing block. In general, generated boxes
  act as containing blocks for descendant boxes; we say that a box "establishes"
  the containing block for its descendants. The phrase "a box&#39;s containing
  block" means "the containing block in which the box lives," not the one it
  generates.


  Each box is given a position with respect to its containing block, but it is
  not confined by this containing block; it may
  <a href="http://www.w3.org/TR/CSS2/visufx.html#overflow">overflow</a>.


<h3 id="def-cb">
Definition of <dfn lt="containing block">containing block</dfn></h3>

  The position and size of an element&#39;s box(es) are sometimes computed relative
  to a certain rectangle, called the containing block of the element. The containing
  block of a ''position/static'' or
  ''position/relative'' element is defined
  in the Box Model [[!CSS3BOX]]. The containing block of a
  ''position/sticky'' element is the same as for a
  ''position/relative'' element. For
  ''position/fixed'',
  ''position/absolute'',
  ''position/center'' and
  ''position/page'', it is defined as follows:

  <ol>
    <li>
    If the element has ''position: fixed'', the containing block is established by the
    <a>viewport</a> in the case of continuous media or the page area in
    the case of paged media.

    <li>
    If the element has ''position: absolute'',
    the containing block is established by the nearest ancestor with a 'position'
    other than ''position/static'', in the following way:
    <ol>
      <li>
      In the case that the ancestor is block-level, the containing block is formed
      by the padding edge of the ancestor.

      <li>
      In the case that the ancestor is inline-level, the containing block depends
      on the 'direction' property of the ancestor:
      <ol>
        <li>
          If the 'direction' is ''ltr'', the top and left of the containing
          block are the top and left content edges of the first box generated
          by the ancestor, and the bottom and right are the bottom and right
          content edges of the last box of the ancestor.

        <li>
        If the 'direction' is ''rtl'', the top and right are the
        top and right edges of the first box generated by the ancestor, and
        the bottom and left are the bottom and left content edges of the
        last box of the ancestor.
        <p class="note">
          Note, in some cases when a line wraps it may seem as if the left
          and right positions are swapped.
        </p>
      </ol>
    </ol>
    <li>
    I the element has ''position: center'', the containing block is established
    by the nearest ancestor with a 'position' other than ''position/static'', in the following way:
    <ol>
      <li>
      In the case that the ancestor is block-level, the containing block is formed
      by the padding edge of the ancestor.

      <li>
      In the case that the ancestor is inline-level, the containing block depends
      on the content edges of the boxes generated by the ancestor. The top most,
      right most, bottom most and left most content edges of all the boxes
      generated by the ancestor determine the edges for the containing block.
    </ol>

    <li>
    If the element has ''position: page''
    and the element is within paged media or within a rerion, the containing
    block is the <a>initial containing block</a>. This is the <a>viewport</a>
    or the page area when in paged media. In the case of CSS Regions
    [[!CSS3-REGIONS]] this is the individual region. When a page positioned
    element is not in paged media or a region the containing block is determined
    per the absolute positioning model.

    <li>
    If there is no such ancestor, the containing block is the <a>initial
    containing block</a>.
  </ol>

  In paged media, an absolutely positioned element is positioned relative to its
  containing block ignoring any page breaks (as if the document were continuous). The
  element may subsequently be broken over several pages.

  For absolutely positioned content 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
  which has already been rendered for printing, printers may place the content:

  <ul>
    <li>on the current page,</li>
    <li>on a subsequent page, or</li>
    <li>may omit it altogether.</li>
  </ul>

<p class="note">
  Note, a block-level element that is split over several pages may have a different width
  on each page and that there may be device-specific limits.
</p>

<div class="example">

  With no positioning, the containing blocks (C.B.) in the following document:

<pre><code class="html">
&lt;!DOCTYPE html&gt;
&lt;html&gt;
    &lt;head&gt;
        &lt;title&gt;Illustration of containing blocks&lt;/title&gt;
    &lt;/head&gt;
    &lt;body id="body"&gt;
        &lt;div id="div1"&gt;
        &lt;p id="p1"&gt;This is text in the first paragraph...&lt;/p&gt;
        &lt;p id="p2"&gt;This is text &lt;em id="em1"&gt; in the
        &lt;strong id="strong1"&gt;second&lt;/strong&gt; paragraph.&lt;/em&gt;&lt;/p&gt;
        &lt;/div&gt;
    &lt;/body&gt;
&lt;/html&gt;
</code></pre>

  are established as follows:

<table class="cb-example-table">
  <tr><th>For box generated by </th>
  <th>C.B. is established by</th></tr>
  <tr><td>html</td><td>initial C.B. (UA-dependent)</td></tr>
  <tr><td>body</td><td>html</td></tr>
  <tr><td>div1</td><td>body</td></tr>
  <tr><td>p1</td><td>div1</td></tr>
  <tr><td>p2</td><td>div1</td></tr>
  <tr><td>em1</td><td>p2</td></tr>
  <tr><td>strong1</td><td>p2</td></tr>
</table>

  If we position "div1":

<pre><code class="css">
#div1 { position: absolute; left: 50px; top: 50px }
</code></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><code class="css">
#div1 { position: absolute; left: 50px; top: 50px }
#em1  { position: absolute; left: 100px; top: 100px }
</code></pre>

  the table of containing blocks becomes:

  <table class="cb-example-table">
    <tr><th>For box generated by</th>
    <th>C.B. is established by</th></tr>
    <tr><td>html</td><td>initial C.B. (UA-dependent)</td></tr>
    <tr><td>body</td><td>html</td></tr>
    <tr><td>div1</td><td>initial C.B.</td></tr>
    <tr><td>p1</td><td>div1</td></tr>
    <tr><td>p2</td><td>div1</td></tr>
    <tr><td>em1</td><td>div1</td></tr>
    <tr><td>strong1</td><td>em1</td></tr>
  </table>

  By positioning "em1", its containing block becomes the nearest positioned ancestor box
  (i.e., that generated by "div1").
</div>

<!-- End section: Containing blocks -->

<h2 id="nf">
<dfn>Normal Flow</dfn></h2>

  Boxes in the normal flow belong to a formatting context, which may be block or
  inline, but not both simultaneously. See the CSS Basic Box Model module
  [[!CSS3BOX]] for further details about <a href="http://www.w3.org/TR/CSS2/visuren.html#normal-flow">normal flow</a>.

<!-- End section: Normal Flow -->

<h2 id="floats">
<dfn lt="float|floated box|floating box">Floats</dfn></h2>

  A float is a box that is positioned within content, often left or right on the
  current line. The most interesting characteristic of a float (or "floated" or
  "floating" box) is that inline-level content may flow along its side (or be
  prohibited from doing so by the 'clear' property) or around the floated box.
  See the CSS 2.1 [[!CSS21]]<!--CSS Floats module [[!CSS3FLOATS]]--> for details
  about <a href="http://www.w3.org/TR/CSS2/visuren.html#floats">floating boxes</a>.

<!-- End section: Floats -->

<h2 id="pos-sch">
<dfn>Positioning schemes</dfn></h2>

  In CSS, a box may be laid out according to three positioning schemes:
  <ol>
    <li>
    Normal flow<br>
    In CSS, <a>normal flow</a> includes <a href="http://www.w3.org/TR/CSS2/visuren.html#block-formatting">block formatting</a> of block-level boxes,
    <a href="http://www.w3.org/TR/CSS2/visuren.html#inline-formatting">inline formatting</a> of inline-level boxes,
    and <a lt="relative positioning">relative</a> and <a lt="sticky positioning">sticky</a> positioning of block-level and inline-level boxes.

    <li>
    Floats<br>
    In the <a>float</a> model, a box is first laid out according to the <a>normal flow</a>, then taken
    out of the flow and positioned, typically to the left or right. Content may flow
    along the side of a <a>float</a>.

    <li>
    Absolute positioning<br>
    In the absolute positioning model, a box is removed from the <span>normal
    flow</span> entirely (it has no impact on later siblings) and assigned a
     position with respect to a <a>containing block</a>.
  </ol>

  An element is called <em>out-of-flow</em> if it is <a>floated</a>, absolutely positioned, or is the root element. An element is called
  <em>in-flow</em> if it is not out-of-flow. The <em>flow of an element</em>
  A is the set consisting of A and all in-flow elements whose nearest
  out-of-flow ancestor is A.

<h3 id="rel-pos">
Relative positioning</h3>

  Once a box has been laid out according to the <a>normal flow</a> or
  <a>floated</a>, it may be offset relative to this position.
  This is called relative positioning. Offsetting a box (B1) in this way has no
  effect on the box (B2) that follows: B2 is given a position as if B1 were not
  offset and B2 is not re-positioned after B1&#39;s offset is applied. This
  implies that relative positioning may cause boxes to overlap. However, if
  relative positioning causes an 'overflow: auto' or 'overflow: scroll' box to
  have overflow, the user agent must allow the user to access this content (at
  its offset position), which, through the creation of a scrolling mechanism,
  may affect layout.

  A relatively positioned box keeps its <a>normal flow</a> size, including
  line breaks and the space originally reserved for it.

  A relatively positioned box establishes a new a new <a>containing block</a> for
  absolutely positioned descendants. (This is a common use of relatively positioned
  boxes.) The section on <a>containing blocks</a>
  explains when a relatively positioned box establishes a new <a>containing block</a>.

  For relatively positioned elements, 'left' and 'right' move the box(es) horizontally,
  without changing their size. 'Left' moves the boxes to the right, and 'right' moves
  them to the left. Since boxes are not split or stretched as a result of 'left' or
  'right', the used values are always: left = -right.

  If both 'left' and 'right' are ''auto'' (their initial values), the used values
  are ''0'' (i.e., the boxes stay in their original position).

  If 'left' is ''auto'', its used value is minus the value of 'right' (i.e., the
  boxes move to the left by the value of 'right').

  If 'right' is specified as ''auto'', its used value is minus the value of 'left'.

  If neither 'left' nor 'right' is ''auto'', the position is over-constrained, and
  one of them has to be ignored. If the 'direction' property of the <a>containing
  block</a> is ''ltr'', the value of 'left' wins and 'right' becomes -'left'.
  If 'direction' of the containing block is ''rtl'', 'right' wins and 'left' is
  ignored.

<div class="example">

  The following three rules are equivalent:

<pre><code class="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 }
</code></pre>
</div>

  The 'top' and 'bottom' properties move relatively positioned element(s) up or
  down without changing their size. 'Top' moves the boxes down, and 'bottom' moves
  them up. Since boxes are not split or stretched as a result of 'top' or 'bottom',
  the used values are always: top = -bottom.

  If 'top' and 'bottom' are ''auto'', their used values are both ''0''.

  If one of them, 'top' or 'bottom', is ''auto'', the ''auto'' value becomes the
  negative of the other.

  If neither 'top' and 'bottom' is ''auto'', 'bottom' is ignored (i.e., the used
  value of 'bottom' will be minus the value of 'top').

  <p class="note">
    Note, Although relative positioning could be used as a form of superscripting
    and subscripting, the line height is not automatically adjusted to take the
    positioning into consideration. See the description of <a href="http://www.w3.org/TR/CSS2/visudet.html#line-height">line height calculations</a> for more
    information.
  </p>

  Examples of relative positioning are provided in the section comparing <a>normal
  flow</a>, <a>floats</a>, and absolute positioning.

<!-- End section: Relative positioning  -->

<h3 id="sticky-pos">
Sticky positioning</h3>

  A <a>stickily positioned</a> box is positioned similarly to a <a>relatively
  positioned</a> box, but the offset is computed with reference to the nearest
  ancestor with a <a>scrolling box</a>, or the viewport if no ancestor has
  a <a>scrolling box</a>.

  Specifically, once a box has been laid out according to the <a>normal flow</a>
  or <a>floated</a>, its sticky offset is computed as described
  below. Offsetting a box (B1) in this way has no effect on the box (B2) that
  follows: B2 is given a position as if B1 were not offset and B2 is not re-positioned
  after B1’s offset is applied. This implies that sticky positioning may cause
  boxes to overlap. However, if sticky positioning causes an ''overflow: auto'' or
  ''overflow: scroll'' box to have overflow, the user agent must allow the user to
  access this content (at its offset position), which, through the creation of a
  scrolling mechanism, may affect layout.

  A stickily positioned box keeps its <a>normal flow</a> size, including  line
  breaks and the space originally reserved for it.

  A stickily positioned box establishes a new a new <a>containing block</a> for
  absolutely positioned descendants, just as relative positioning does. The section
  on <a>containing blocks</a> explains when a
  stickily positioned box establishes a new <a>containing block</a>.

  For stickily positioned elements, 'left', 'right', 'top' and 'bottom' are
  offsets from the respective edges of its flow box which are used to constrain
  the element's offset. Percentage values of 'left' and 'right' refer to the
  width of its flow box; percentage values of 'top' and 'bottom' refer to the
  height of its flow box.

  <p class="issue">
    Describe which element font-size-relative units are resolved against
  </p>

  The offset of a stickily positioned box is computed as follows:

<ol>
  <li>
  A rectangle is computed relative to the containing block of the stickily
  positioned element, by insetting its flow box rectangle on each side by offsets
  computed from the 'left', 'right', 'top' and 'bottom' properties of the stickily
  positioned element.
  <p class="issue">
    Say what happens if this rectangle is empty
  </p>

  <li>
  The intersection is taken between the resulting rectangle, and the containing
  block of the stickily positioned element. The result, termed the the
  <em>sticky-constraint rectangle</em>, is a rectangle used to constrain the
  location of the stickily positioned element.

  <li>
  If the stickily positioned element's 'top' style is not ''auto'', and the stickily
  positioned element projects above the top of the <em>sticky-constraint
  rectangle</em>, the stickily positioned element is moved down until it is fully
  contained in the <em>sticky-constraint rectangle</em>.

  <li>
  If the stickily positioned element's 'bottom' style is not ''auto'', and the stickily
  positioned element projects below the bottom of the <em>sticky-constraint
  rectangle</em>, the stickily positioned element is moved up until it is fully
  contained in the <em>sticky-constraint rectangle</em>.

  <li>
  If the stickily positioned element's 'left' style is not ''auto'', and the stickily
  positioned element projects outside the left of the <em>sticky-constraint
  rectangle</em>, the stickily positioned element is moved right until it is fully
  contained in the <em>sticky-constraint rectangle</em>.

  <li>
  If the stickily positioned element's 'right' style is not ''auto'', and the stickily
  positioned element projects outside the right of the <em>sticky-constraint
  rectangle</em>, the stickily positioned element is moved left until it is fully
  contained in the <em>sticky-constraint rectangle</em>.
</ol>

  When computing containement of the stickily positioned element within its
  containing block, margins on the stickily positioned element are taken into
  account.

  <p class="issue">
    Say what happens if it already overflows the containing block
  </p>
  <p class="issue">
    Do marins collapse between the stickily positioned element and its containing
    block element?
  </p>

  Intersection between the stickily positioned element and the bottom of the
  <em>sticky-constraint rectangle</em> limits movement in any direction, so the
  offset never pushes the stickily positioned element outside of its containing
  block. However, when the element is free to move within its containing block
  as the page is scrolled, it appears to be pinned to the relevant flow root
  edges, similarly to a fixed position element.

  <p class="note">
    Note that a stickily positioned element with non-auto 'top' style and auto
    'bottom' style will only ever be pushed down by sticky positioning; it will
    never be offset upwards.
  </p>
  <p class="note">
    Multiple stickily positioned elements in the same container are offset
    independently. Sticky position offsets may cause them to overlap.
  </p>
  <p class="issue">
    Does the margin on the stickily positioned element affect its distance from
    the flow root edge?
  </p>
  <p class="issue">
    Sticky positioning should really be defined in terms of the nearest scrollable
    ancestor, but there is currently no such term defined elsewhere in CSS. CSSOM
    View refers to "scrolling boxes." CSS Overflow has yet to pull in the relevant
    text from CSS Box, and CSS Box has an old, confusing definition of "flow root"
    which is almost (but probably not quite) what we want here. This spec refers
    to "flow root," since that's the closest thing currently specced somewhere,
    but this is not optimal.
  </p>
<div class=example>
  The following example is the same as the previous one, but now it is explained:

  <pre>EM { font-style: italic }</pre>
</div>
<!-- End section: Sticky positioning  -->

<h3 id="abs-pos">
Absolute positioning</h3>

  In the absolute positioning model, a box is explicitly offset with respect to
  its <a>containing block</a>. It is removed from the <a>normal flow</a> entirely
  (it has no impact on later siblings). An absolutely positioned box establishes
  a new <a>containing block</a> for normal flow children and absolutely (but not
  fixed or page) positioned descendants. However, the contents of an absolutely
  positioned element do not flow around any other boxes. They may obscure the
  contents of another box (or be obscured themselves), depending on the stack
  levels of the overlapping boxes.

  References in this specification to an absolutely positioned element (or its
  box) imply that the element&#39;s 'position' property has the value
  ''absolute'' or ''fixed''.
<!-- End section: Absolute positioning -->

<h3 id="center-pos">
Center positioning</h3>

  In the center positioning model, a box is explicitly centered with respect to
  its <a>containing block</a>. It is removed from the <a>normal flow</a> entirely
  (it has no impact on later siblings). A center positioned box establishes a new
  <a>containing block</a> for <a>normal flow</a> children and absolutely (but not
  fixed or page) positioned descendants. However, the contents of an center
  positioned element do not flow around any other boxes. They may obscure the
  contents of another box (or be obscured themselves), depending on the stack
  levels of the overlapping boxes.

  References in this specification to an <a>center positioned element</a>
  (or its box) imply that the element&#39;s 'position' property has the value
  ''center''.
<!-- End section: Center positioning -->

<h3 id="page-positioning">
Page positioning</h3>

  In the page positioning model, a box is explicitly offset with respect to its
  <a>containing block</a>. It is removed from the <a>normal flow</a> entirely
  (it has no impact on later siblings). A page positioned box establishes a new
  <a>containing block</a> for <a>normal flow</a> children and absolutely (but
  not fixed or other page) positioned descendants. However, the contents of a page
  positioned element do not flow around any other boxes. They may obscure the
  contents of another box (or be obscured themselves), depending on the stack
  levels of the overlapping boxes.

  For <a href="http://www.w3.org/TR/CSS2/page.html">paged media</a>, boxes with
  page positions are only generated on the initial page where the page position
  element exists. Boxes with page position that are larger than the page area
  are clipped and the remaining part of the box is placed on the following page.

  References in this specification to a <a>page positioned element</a>
  (or its box) imply that the element’s 'position' property has the value
  ''page''.
<!-- End section: Page positioning -->

<h3 id="fixed-pos">
Fixed positioning</h3>

  Fixed positioning is similar to absolute positioning. The only difference is
  that for a fixed positioned box, the <a>containing block</a> is established
  by the <a>viewport</a>. For <a href="http://www.w3.org/TR/CSS2/media.html#continuous-media-group">continuous media</a>,
  fixed boxes do not move when the document is scrolled. In this respect, they are similar
  to <a href="http://www.w3.org/TR/CSS2/colors.html#background-properties">fixed background images</a>.
  For <a href="http://www.w3.org/TR/CSS2/media.html#paged-media-group">paged
  media</a>, boxes with fixed positions are repeated on every page. This is useful
  for placing, for instance, a signature at the bottom of each page. Boxes with
  fixed position that are larger than the page area are clipped. Parts of the
  fixed position box that are not visible in the <span>initial containing
  block</span> will not print.

<div class="example">

  Authors may use fixed positioning to create the following presentation:

  <figure>
    <img src="images/frame.png" alt="Example of frame layout">
    <span class="dlink"><a id="img-frame" href="images/longdesc/frame-desc.html" title="Long description for frame layout example">\[D]</a></span>
    <span class="slink">&nbsp;&nbsp;&nbsp;<a id="source-frame" href="source/frame-src.html" title="Source code for frame layout example">\[S]</a></span>

  </div>

  This might be achieved with the following HTML document and style rules:
<pre><code class="html">
&lt;!DOCTYPE html&gt;
&lt;html&gt;
  &lt;head&gt;
    &lt;title&gt;A frame document with CSS&lt;/title&gt;
    &lt;style type="text/css" media="screen"&gt;
      body { height: 8.5in } /* Required for percentage heights below */
      #header {
        position: fixed;
        width: 100%;
        height: 15%;
        top: 0;
        right: 0;
        bottom: auto;
        left: 0;
      }
      #sidebar {
        position: fixed;
        width: 10em;
        height: auto;
        top: 15%;
        right: auto;
        bottom: 100px;
        left: 0;
      }
      #main {
        position: fixed;
        width: auto;
        height: auto;
        top: 15%;
        right: 0;
        bottom: 100px;
        left: 10em;
      }
      #footer {
        position: fixed;
        width: 100%;
        height: 100px;
        top: auto;
        right: 0;
        bottom: 0;
        left: 0;
      }
    &lt;/style&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;div id="header"&gt; ...  &lt;/div&gt;
    &lt;div id="sidebar"&gt; ...  &lt;/div&gt;
    &lt;div id="main"&gt; ...  &lt;/div&gt;
    &lt;div id="footer"&gt; ...  &lt;/div&gt;
  &lt;/body&gt;
&lt;/html&gt;
</code></pre>
</div>

<!-- End section: Fixed positioning -->
<!-- End section: Positioning schemes -->

<h3 id="position-property">
Choosing a positioning scheme: 'position' property</h3>

  The 'position' property determines which of the positioning algorithms is used to
  calculate the position of a box.

  <pre class="propdef">
  Name: position
  Value: static | relative | absolute | sticky | center | page | fixed
  Initial: static
  Applies to: all elements except table-column-group and table-column
  Inherited: no
  Animatable: no
  Percentages: N/A
  Media: visual
  Computed value: specified value
  Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
  </pre>

  The values of this property have the following meanings:
<dl dfn-for="position" dfn-type="value">
  <dt><dfn>static</dfn></dt>
  <dd>
    The box is a normal box, laid out according to the <a>normal flow</a>. The
    'top', 'right', 'bottom', and 'left' properties do not apply.

  <dt><dfn>relative</dfn></dt>
  <dd>
    The box&#39;s position is calculated according to the <a>normal flow</a> (this
    is called the position in <a>normal flow</a>). Then the box is offset relative
    to its normal position and in all cases, including table elements, does not affect
    the position of any following boxes. When a box B is relatively positioned, the
    position of the following box is calculated as though B were not offset. The effect
    of ''position: relative'' on table elements is defined as follows:
    <ul>
      <li>
      table-row-group, table-header-group, table-footer-group and table-row offset
      relative to its normal position within the table. If table-cells span
      multiple rows, only the cells originating in the <a>relative positioned</a> row is
      offset.

      <li>
      table-column-group, table-column do not offset the respective column and has
      no visual affect when 'position': ''relative'' is applied.

      <li>
      table-caption and table-cell offset relative to its normal position within
      the table. If a table cell spans multiple columns or rows the full spanned
      cell is offset.
    </ul>

  <dt><dfn>absolute</dfn></dt>
  <dd>
    The box&#39;s position (and possibly size) is specified with the 'top', 'right',
    'bottom', and 'left' properties. These properties specify offsets with respect
    to the box&#39;s <a>containing block</a>. Absolutely positioned boxes
    are taken out of the normal flow. This means they have no impact on the layout
    of later siblings. Though absolutely positioned boxes may have margins, those
    margins do not <a href="http://www.w3.org/TR/CSS2/box.html#collapsing-margins">
    collapse</a> with any other margins.

  <dt><dfn>sticky</dfn></dt>
  <dd>
    The box&#39;s position is calculated according to the <a>normal flow</a>
    (this is called the position in <a>normal flow</a>). Then the box is offset
    relative to its flow root and containing block and in all cases, including
    table elements, does not affect the position of any following boxes. When a
    box B is stickily positioned, the position of the following box is calculated
    as though B were not offset. The effect of 'position': ''sticky'' on table
    elements is the same as for 'position': ''relative''

  <dt><dfn>center</dfn></dt>
  <dd>
    The box&#39;s position (and possibly size) is specified with the 'top', 'right',
    'bottom', and 'left' properties. The box is vertically and horizontally centered
    within its <a>containing block</a> and these properties specify offsets with
    respect to the box&#39;s centered position within its <a>containing block</a>.
    Center positioned boxes are taken out of the normal flow. This means they have
    no impact on the layout of later siblings. Though center positioned boxes may
    have margins, those margins do not <a href="http://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>
    with any other margins.

  <dt><dfn>page</dfn></dt>
  <dd>
    The box&#39;s position is calculated according to the "absolute" model.
    <ul>
      <li>
      In the case of paged media or when inside a region box the box&#39;s
      <a>containing block</a> is always the <span>initial containing
      block</span>.

      <li>
      Otherwise, the <a>containing block</a> is determined per the
      "absolute" model.
    </ul>

    As with the "absolute" model, the box&#39;s margins do not collapse with any
    other margins. In the case of the print media type, the box is rendered only
    on the initial page or region where the page positioned element originated.
    User agents may paginate the content of paged boxes.

    <p class="note">
      Note, that CSS Regions are also <a>initial containing blocks</a>,
      in accordance with 'flow-into' property of the
      CSS Regions Module [[!CSS3-REGIONS]].
    </p>

  <dt><dfn>fixed</dfn></dt>
  <dd>
    The box&#39;s position is calculated according to the "absolute" model, but
    in addition, the box is fixed with respect to some reference. As with the
    "absolute" model, the box&#39;s margins do not collapse with any other margins.
    In the case of handheld, projection, screen, tty, and tv media types, the box
    is fixed with respect to the <a>viewport</a> and does not move when scrolled.
    In the case of the print media type, the box is rendered on every page, and
    is fixed with respect to the page box, even if the page is seen through a
    <a>viewport</a> (in the case of a print-preview, for example). For other media
    types, the presentation is undefined. 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="http://www.w3.org/TR/CSS2/media.html#at-media-rule">'@media'</a>
    rule, as in:
    <div class="example">
<pre><code class="css">
@media screen {
    h1#first { position: fixed }
}
@media print {
    h1#first { position: static }
}
</code></pre>
    </div>
    User agents must not paginate the content of fixed boxes.

    <p class="note">
      Note that user agents may print invisible content in other ways. See
      <a href="http://www.w3.org/TR/CSS2/page.html#outside-page-box">"Content outside the page box"</a>.
    </p>
  </dd>
</dl>

  In previous versions of CSS user agents may treat position ''fixed'' as ''static''
  on the root element. This specification removes that option and it is now required
  that newer user agents treat ''fixed'' value
872
 on the root element as defined for
  all other elements.
<!-- End section: Choosing a positioning scheme: 'position' property -->

<h3 id="box-offsets-trbl">
Box offsets: 'top', 'right', 'bottom', 'left'</h3>

  An element is said to be <a>positioned</a> if its 'position' property has a value other than ''static''.
  Positioned elements generate positioned boxes, and may be laid out according
  to the following four physical properties:

  <pre class="propdef">
  Name: top
  Value: auto | &lt;length&gt; | &lt;percentage&gt;
  Initial: auto
  Applies to: positioned elements
  Inherited: no
  Animatable: &lt;length&gt;, &lt;percentage&gt;
  Percentages: refer to height of <a>containing block</a>
  Media: visual
  Computed value: For 'position': ''relative'', see Relative positioning.<br/> For 'position': ''sticky'', see Sticky positioning.<br/> For 'position': ''static'', ''auto''.<br/> Otherwise: if specified as a '&lt;length&gt;', the corresponding absolute length; if specified as a '&lt;percentage&gt;', the specified value; otherwise, ''auto''.
  Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
  </pre>

  This property specifies how far an absolutely positioned box&#39;s top margin
  edge is offset below the top edge of the box&#39;s <a>containing block</a>.

  For relatively positioned boxes, the offset is with respect to the top edge of
  the box itself (i.e., the box is given a position in the <a>normal flow</a>,
  and then offset from that position according to this property).

  For stickily positioned boxes, the value is used to compute the <em>sticky-constraint
  rectangle</em> as described in Sticky positioning.

  For absolutely positioned and center positioned elements whose <a>containing
  block</a> is based on a block-level element, this property is an offset from
  the padding edge of that element.

  For page positioned boxes, within paged media or regions, the offset is with
  respect to the top edge of the <span>initial containing block</span>; otherwise
  for page positioned boxes same as for absolutely positioned boxes.

  <pre class="propdef">
  Name: right
  Value: auto | &lt;length&gt; | &lt;percentage&gt;
  Initial: auto
  Applies to: positioned elements
  Inherited: no
  Animatable: &lt;length&gt;, &lt;percentage&gt;
  Percentages: refer to height of <a>containing block</a>
  Media: visual
  Computed value: For 'position': ''relative'', see Relative positioning.<br/> For 'position': ''sticky'', see Sticky positioning.<br/> For 'position': ''static'', ''auto''.<br/> Otherwise: if specified as a '&lt;length&gt;', the corresponding absolute length; if specified as a '&lt;percentage&gt;', the specified value; otherwise, ''auto''.
  Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
  </pre>

  Similar to 'top', but specifies how far a box&#39;s right margin edge is offset
  to the left of the right edge of the box&#39;s <a>containing block</a>.

  For relatively positioned boxes, the offset is with respect to the right edge
  of the box itself (i.e., the box is given a position in the <a>normal flow</a>,
  and then offset from that position according to this property).

  For stickily positioned boxes, the value is used to compute the <em>sticky-constraint
  rectangle</em> as described in Sticky positioning.

  For absolutely positioned and center positioned elements whose <a>containing
  block</a> is based on a block-level element, this property is an offset from
  the padding edge of that element.

  For page positioned boxes, within paged media or regions, the offset is with
  respect to the right edge of the <span>initial containing block</span>; otherwise
  for page positioned boxes same as for absolutely positioned boxes.

  <pre class="propdef">