csswg-drafts/css4-background/Overview.src.html at 4c2c2dd65b6a2ec5fb6f92f07c13300ff7a7de30 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC '-//W3C//DTD HTML 4.01//EN'
  'http://www.w3.org/TR/html4/strict.dtd'>

<html lang="en">
<head>
  <title>CSS Backgrounds and Borders Module Level 4</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <style type="text/css">
    .prod { font-family: inherit; font-size: inherit }
    pre.prod { white-space: pre-wrap; margin: 1em 0 1em 2em }
    code { font-size: inherit; }
    #box-shadow-samples td { background: white; color: black; }
  </style>
  <link rel="stylesheet" type="text/css"
  href="http://www.w3.org/StyleSheets/TR/W3C-WD.css">
</head>

<body>

<div class="head">
<!--logo-->

<h1>CSS Backgrounds and Borders Module Level 4</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:
    <dd><!-- <a href="http://www.w3.org/TR/[YEAR]/CR-[SHORTNAME]-[CDATE]">[VERSION]</a> -->
     <a href="http://dev.w3.org/csswg/css3-background">http://dev.w3.org/csswg/css3-background</a>
  <dt>Latest version:
    <dd><a
      href="http://www.w3.org/TR/css3-background">[LATEST]</a>
  <dt>Previous versions:
    <dd><a href="http://www.w3.org/TR/2010/WD-css3-background-20100612/">http://www.w3.org/TR/2010/WD-css3-background-20100612/</a>
  <dt id="editors-list">Editors:
    <dd><a href="mailto:bert@w3.org">Bert Bos</a> (<a
      href="http://www.w3.org/">W3C</a>)
    <dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Invited Expert)
    <dd><a href="mailto:brad.kemper@gmail.com">Brad Kemper</a> (Invited Expert)
</dl>

<!--copyright-->

<hr title="Separator for header">
</div>


<h2 class="no-num no-toc" id="abstract">
Abstract</h2>

  <p>CSS is a language for describing the rendering of structured documents
  (such as HTML and XML) on screen, on paper, in speech, etc.
  This draft contains the features of CSS relating to the borders and backgrounds
  of boxes on the page. It includes and extends the functionality of
  CSS Backgrounds and Borders Level&nbsp;3. [[CSS3BACKGROUND]]
  The main extensions compared to level&nbsp;3 are shaping ('corner-shape')
  and clipping borders ('border-clip'), logical background positions
  ('background-position'), and the ''extend'' ability of 'background-repeat'.

<h2 class="no-num no-toc" id="status">
Status of this document</h2>
<!--status-->
  <!--
  <p>The following features are at-risk and may be dropped at the
  end of the CR period if there has not been enough interest from
  implementers: [...]
  -->

  <p>The CSS WG maintains an <a href="http://www.w3.org/Style/CSS/Tracker/products/11">issues
  list</a> for this module. A <a href="http://www.w3.org/Style/CSS/Test/">test
  suite</a> and implementation report for CSS Backgrounds and Borders Level 4
  will be developed during the Candidate Recommendation phase, which will last
  a minimum of three months<!--, i.e., at least until 15 May 2011-->. See the
  section <a href="#exit">“CR exit criteria”</a> for more details.

<h2 class="no-num no-toc" id="contents">Table of contents</h2>
<!--toc-->

<h2 id="introduction">
Introduction</h2>

  <p><em>This section is not normative.</em>

  <p>When elements are rendered according to the
  <a href="http://www.w3.org/TR/CSS21/box.html">CSS box model</a> [[!CSS21]],
  each element is either not displayed at all, or formatted as one
  or more rectangular boxes. Each box has a rectangular content
  area, a band of padding around the content, a border around the
  padding, and a margin outside the border. (The margin may actually be
  negative, but margins have no influence on the background and border.)

  <div class="figure">
    <p><img src="box" alt="Diagram of a typical box, showing the
    content, padding, border and margin area
1898
s">

    <p class="caption">The various areas and edges of a typical box.
    (This diagram is explained in the CSS2.1 Box Model chapter [[!CSS21]].)
  </div>

  <p>The properties of this module deal with the decoration of the border
  area and with the background of the content, padding and border areas.
  Additionally the box may be given a "drop-shadow" effect with the
  'box-shadow' property.

  <p>If an element is broken into multiple boxes, 'box-decoration-break'
  defines how the borders and background are divided over the various boxes.
  (An element can result in more than one box if it is broken at the
  end of a line, at the end of a column or at the end of a page; and
  continued in the next line, column or page.)

  <p>The relative stacking order of backgrounds, borders, and shadows
  is given in this module. For how these layers interact with other
  rendered content, see Appendix E “Elaborate description of Stacking
  Contexts” in [[!CSS21]].

<h2 id="values">
Values</h2>

  <p>This specification follows the
  <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
  definition conventions</a> from [[!CSS21]] and the value type definitions
  from CSS Values and Units Level 3 [[!CSS3VALUES]].</p>

<h2 id="backgrounds">
Background properties</h2>

  <p>Each box has a background layer that may be fully transparent (the
  default), or filled with a color and/or one or more images. The background
  properties specify what color ('background-color')
  and images ('background-image') to use,
  and how they are sized, positioned, tiled, etc.

  <p>The background properties are not inherited, but the parent box's
  background will shine through by default because of the initial
  'transparent' value on 'background-color'.

<h3 id="layering">
Layering multiple background images</h3>

  <p>The background of a box can have multiple <span class=index>layers</span>.
  The number of layers is determined by the number of comma-separated
  values in the 'background-image' property.
  Note that a value of 'none' still creates layer.

  <p>Each of the images is sized, positioned, and tiled according to
  the corresponding value in the other background properties. The lists
  are matched up from the first value: excess values at the end are not
  used. If a property doesn't have enough comma-separated values to match
  the number of layers, the <span>UA</span> must calculate its used
  value by repeating the list of values until there are enough.

  <div class="example">
    <p>For example, this set of declarations:
    <pre>
<!-- -->background-image: url(flower.png), url(ball.png), url(grass.png);
<!-- -->background-position: center center, 20% 80%, top left, bottom right;
<!-- -->background-origin: border-box, content-box;
<!-- -->background-repeat: no-repeat;
    </pre>
    <p>has exactly the same effect as this set with the extra position
    dropped and the missing values for 'background-origin' and
    'background-repeat' filled in (emphasized for clarity):
    <pre>
<!-- -->background-image: url(flower.png), url(ball.png), url(grass1.png);
<!-- -->background-position: center center, 20% 80%, top left;
<!-- -->background-origin: border-box, content-box<strong>, border-box</strong>;
<!-- -->background-repeat: no-repeat<strong>, no-repeat, no-repeat</strong>;
    </pre>
  </div>

  <p>The first image in the list is the layer closest to the user,
  the next one is painted behind the first, and so on. The background
  color, if present, is painted below all of the other layers.

  <p class="note">Note that the <a href="#border-images">border-image properties</a>
  can also define a background image, which, if present, is painted on top of
  the background created by the background properties.

<h3 id="the-background-color">
The 'background-color' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-color</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;color&gt;</var>
    <tr>
      <th>Initial:
      <td>transparent
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>the computed color(s)
  </table>

  <p>This property sets the <span class=index
  title="color!!background">background color</span> of an element. The
  color is drawn behind any background images.

  <div class="example">
  <p>Example:
  <pre>h1 { background-color: #F00 } /* Sets background to red. */</pre>
  </div>

  <p>The background color is clipped according to the 'background-clip'
  value associated with the bottom-most background image.

<h3 id="the-background-image">
The 'background-image' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-image</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;bg-image&gt;</var>
      [ , <var>&lt;bg-image&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>as specified, but with URIs made absolute
  </table>

  <p>This property sets the background image(s) of an element. Images
  are drawn with the first specified one on top (closest to the user)
  and each subsequent image behind the previous one.
  Where

  <pre class=prod><dfn><var>&lt;bg-image&gt;</var></dfn> = <var>&lt;image&gt;</var> | none</pre>

  <p>A value of <dfn title="''none''!!'background-image' value">''none''</dfn>
  counts as an image layer but draws
  nothing. An image that is empty (zero width or zero height), that
  fails to download, or that cannot be displayed (e.g., because it is
  not in a supported image format) likewise counts as a layer but
  draws nothing.

  <p>See the section <a href="#layering">“Layering multiple background
  images”</a> for how 'background-image' interacts with other
  comma-separated background properties to form each background image
  layer.

  <p>When setting a background image, authors should also specify a
  ''background-color'' that will preserve contrast with the text for
  when the image is unavailable.

  <p>For accessibility reasons, authors should not use background images
  as the sole method of conveying important information.
  See <a href="http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/F3">Web
  Content Accessibility Guideline F3</a> [[WCAG20]]. Images are not
  accessible in non-graphical presentations, and background images
  specifically might be turned off in high-contrast display modes.

  <p class="note">Note that stylistic foreground images can be provided
  in CSS with the
  <a href="http://www.w3.org/TR/CSS21/generate.html#content">content</a>
  property. (Semantically-important foreground images should be provided
  in the document markup, e.g. with the &lt;img&gt; tag in HTML.)</p>

  <p class="note"><a href="http://www.w3.org/TR/media-frags/#naming-space">Media fragments</a>
  can be used to display a portion of an image. The
  <a href="http://www.w3.org/TR/css3-images/">CSS Images</a> module
  will provide fallback syntax for image formats and include additional
  controls for image display.

  <div class="example">
  <p>Some examples specifying background images:</p>
  <pre>
<!-- -->body { background-image: url("marble.svg") }
<!-- -->p { background-image: none }
<!-- -->div { background-image: url(tl.png), url(tr.png) }</pre>
  </div>

  <p class=note>Implementations may optimize by not downloading and
  drawing images that are not visible (e.g., because they are behind
  other, fully opaque images).

<h3 id="the-background-repeat">
The 'background-repeat' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-repeat</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;repeat-style&gt;</var> [ , <var>&lt;repeat-style&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>repeat
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>as specified
  </table>

  <p>Specifies how background images are tiled after they have been
  <a href="#the-background-size">sized</a> and <a href="#the-background-position">positioned</a>.
  Where
  <pre class=prod><dfn><var>&lt;repeat-style&gt;</var></dfn> = repeat-x | repeat-y | [repeat | space | round | no-repeat]{1,2}</pre>

  <p>Single values for <var>&lt;repeat-style&gt;</var> have the following
  meanings:

  <dl>
    <dt><dfn>''repeat-x''</dfn>
    <dd>Equivalent to ''repeat no-repeat''.

    <dt><dfn>''repeat-y''</dfn>
    <dd>Equivalent to ''no-repeat repeat''.

    <dt>''repeat''
    <dd>Equivalent to ''repeat repeat''.

    <dt>''space''
    <dd>Equivalent to ''space space''

    <dt>''round''
    <dd>Equivalent to ''round round''

    <dt>''no-repeat''
    <dd>Equivalent to ''no-repeat no-repeat''
  </dl>

  <p>If a <var>&lt;repeat-style&gt;</var> value has two keywords, the first
  one is for the horizontal direction, the second for the vertical one,
  as follows:

  <dl>
    <dt><dfn title="''repeat''!!'background-repeat' value">''repeat''</dfn>
    <dd>The image is repeated in this direction as often as needed to cover
    the <span class=index>background painting area</span>.

    <dt><dfn title="''space''!!'background-repeat' value">''space''</dfn>
    <dd>The image is repeated as often as will fit within the
    <span class=index>background positioning area</span> without being
    clipped and then the images are spaced out to fill the area. The
    first and last images touch the edges of the area. If the <span
    class=index>background
    painting area</span> is larger than the background positioning area, then
    the pattern repeats to fill the background painting area.
    The value of 'background-position' for this direction
    is ignored, unless there is not enough space for two copies of the
    image in this direction, in which case only one image is placed and
    'background-position' determines its position in this
    direction.

    <dt><dfn title="''round''!!'background-repeat' value">''round''</dfn>
    <dd>The image is repeated as often as will fit within the
    <span class=index>background positioning area.</span> If it doesn't
    fit a whole number of times, it is rescaled so that it does.
    See the formula under 'background-size'. If the <span
    class=index>background
    painting area</span> is larger than the background positioning area, then
    the pattern repeats to fill the background painting area.

    <dt><dfn title="''no-repeat''!!'background-repeat' value">''no-repeat''</dfn>
    <dd>The image is placed once and not repeated in this direction.
  </dl>

  <p>Unless one of the two keywords is ''no-repeat'', the
  whole background painting area will be tiled, i.e., not just one
  vertical strip and one horizontal strip.

  <div class="example">
    <pre>
<!-- -->body {
<!-- -->  background: white url("pendant.png");
<!-- -->  background-repeat: repeat-y;
<!-- -->  background-position: center;
<!-- -->}</pre>

    <div class="figure">
      <p><img src="bg-repeat" alt="A centered background image, with
      copies repeated up and down the border, padding and content
      areas."><!--<SPAN class="dlink">&nbsp;&nbsp;&nbsp;<A
      name="img-bg-repeat" href="images/longdesc/bg-repeat-desc.html"
      title="Long description for background pattern">[D]</A></SPAN>-->

      <p class="caption">The effect of ''repeat-y'': One copy
      of the background image is centered, and other copies are put above
      and below it to make a vertical band behind the element.
    </div>
  </div>

  <div class=example>
    <pre>
<!-- -->body {
<!-- -->  background-image: url(dot.png) white;
<!-- -->  background-repeat: space
<!-- -->}
<!-- --></pre>

    <div class=figure>
      <p><img src="bg-space" alt="Image of an element with a dotted background">

      <p class=caption>The effect of 'space': the image of a dot is
      tiled to cover the whole background and the images are equally
      spaced.
    </div>
  </div>

  <p>See the section <a href="#layering">“Layering multiple background
  images”</a> for how 'background-repeat' interacts with other
  comma-separated background properties to form each background image
  layer.

<h3 id="the-background-attachment">
The 'background-attachment' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-attachment</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;attachment&gt;</var>  [ , <var>&lt;attachment&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>scroll
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>as specified
  </table>

  <p>If background images are specified, this property specifies whether
  they are fixed with regard to the <em class="index">viewport</em>
  (<dfn>''fixed''</dfn>) or scroll along with the element (<dfn>''scroll''</dfn>)
  or its contents (<dfn>''local''</dfn>). The property's value is given as
  a comma-separated list of &lt;attachment&gt; keywords where

  <pre class=prod><dfn><var>&lt;attachment&gt;</var></dfn> = scroll | fixed | local</pre>

  <dl>
    <dt><dfn>''fixed''</dfn></dt>
      <dd>The background is fixed with regard to the viewport. In paged
        media where there is no viewport, a ''fixed'' background is fixed
        with respect to the <a href="http://www.w3.org/TR/CSS21/page.html#page-box">page box</a>
        and therefore replicated on every page.
        <span class="note">Note that there is only one viewport per view.
        Even if an element has a scrolling mechanism (see  the 'overflow'
        property [[!CSS21]]), a ''fixed'' background doesn't move with the
        element.</span>
    <dt><dfn>''local''</dfn></dt>
      <dd>The background is fixed with regard to the element's contents:
        if the element has a scrolling mechanism, the background scrolls
        with the element's contents. In this case, the background behind
        the element's border (if any) scrolls as well, even though the
        border itself does not scroll with the contents. The UA may,
        however, treat the ''border-box'' value of 'background-clip' as
        ''padding-box'' in cases where 'background-attachment' is ''local''
        and the contents of the element scroll.</dd>
    <dt><dfn>''scroll''</dfn></dt>
      <dd>The background is fixed with regard to the element itself and
        does not scroll with its contents. (It is effectively attached
        to the element's border.)</dd>
  </dl>

  <p>Even if the image is fixed, it is still only visible when it is in
  the <em class=index>background painting area</em> of the element or otherwise
  unclipped. (See <a href="#special-backgrounds"> “The backgrounds of
  special elements”</a> for the cases when background images are not
  clipped.) Thus, unless the image is tiled, it may be invisible.

  <div class="example">
    <p>This example creates an infinite vertical band that remains
    “glued” to the viewport when the element is scrolled.

    <pre>
<!-- -->body {
<!-- -->  background: red url("pendant.gif");
<!-- -->  background-repeat: repeat-y;
<!-- -->  background-attachment: fixed;
<!-- -->}</pre>
  </div>

  <p>User agents that do not support ''fixed'' backgrounds (for
  example due to limitations of the hardware platform) should ignore
  declarations with the keyword ''fixed''. For example:

  <pre class=example>
<!-- -->body {
<!-- -->   /* For all UAs: */
<!-- -->   background: white url(paper.png) scroll;
<!-- -->   /* For UAs that do fixed backgrounds: */
<!-- -->   background: white url(ledger.png) fixed;
<!-- -->}
<!-- -->h1 {
<!-- -->   /* For all UAs: */
<!-- -->   background: silver;
<!-- -->   /* For UAs that do fixed backgrounds: */
<!-- -->   background: url(stripe.png) fixed, white url(ledger.png) fixed;
<!-- -->}</pre>

  <p>See the section <a href="#layering">“Layering multiple background
  images”</a> for how 'background-attachment' interacts with other
  comma-separated background properties to form each background image
  layer.

<h3 id="the-background-position">
The 'background-position' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-position</dfn>
    <tr>
      <th>Value:

      <td><var>&lt;position&gt;</var>
        [ , <var>&lt;position&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>0% 0%
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>refer to size of <span class=index>background positioning
      area</span> <em>minus</em> size of background image; see text
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>If one or two values are specified, for a &lt;length&gt; the
        absolute length, otherwise a percentage. If three or four values
        are specified, two pairs of a keyword plus a length or percentage.
  </table>

  <p>If background images have been specified, this property specifies
  their initial position (after any <a href="#the-background-size">resizing</a>)
  within their corresponding <i>background positioning area</i>.

  <p>Where
  <pre class=prod><dfn><var>&lt;position&gt;</var></dfn> = [
<!-- -->  [ top | bottom ]
<!-- -->|
<!-- -->  [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> | left | center | right ]
<!-- -->  [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> | top | center | bottom ]?
<!-- -->|
<!-- -->  [ center | [ left | right ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ] &amp;&amp;
<!-- -->  [ center | [ top | bottom ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]
<!-- -->]</pre>

  <p>If only one value is specified, the second value is assumed to be
  'center'. If two values are given and at least one value is not a keyword,
  then the first value represents the horizontal position (or offset) and
  the second represents the vertical position (or
  offset). <var>&lt;percentage&gt;</var> and <var>&lt;length&gt;</var>
  values here represent an offset of the top left corner
  of the background image from the top left corner of the background
  positioning area.

  <p>If three or four values are given, then
  each <var>&lt;percentage&gt;</var> or<var>&lt;length&gt;</var>
  represents an offset and must be preceded by a keyword,
  which specifies from which edge the offset is given. For example,
  ''background-position: bottom 10px right 20px'' represents a
  ''10px'' vertical offset up from the bottom edge and a
  ''20px'' horizontal offset leftward from the right edge. If
  three values are given, the missing offset is assumed to be zero.

  <p>Positive values represent an offset <em>inward</em> from the edge of
  the background positioning area. Negative values represent an offset
  <em>outward</em> from the edge of the background positioning area.

  <div class="example">
    <p>The following declarations give the stated (horizontal, vertical)
      offsets from the top left corner:</p>
    <pre>
<!-- -->background-position: left 10px top 15px;   /* 10px, 15px */
<!-- -->background-position: left      top     ;   /*  0
8812
px,  0px */
<!-- -->background-position:      10px     15px;   /* 10px, 15px */
<!-- -->background-position: left          15px;   /*  0px, 15px */
<!-- -->background-position:      10px top     ;   /* 10px,  0px */
<!-- -->background-position: left      top 15px;   /*  0px, 15px */
<!-- -->background-position: left 10px top     ;   /* 10px,  0px */</pre>
  </div>

  <dl>
    <dt><var>&lt;percentage&gt;</var>

    <dd><p>A percentage for the horizontal offset is relative to (<var>width of
      background positioning area</var> - <var>width of background image</var>).
      A percentage for the vertical offset is relative to (<var>height
      of background
      positioning area</var> - <var>height of background image</var>), where the
      size of the image is the size given by 'background-size'.
      <div class=example>
        <p>For example, with a value pair of ''0% 0%'', the upper left
        corner of the image is aligned with the upper left corner of, usually,
        the box's padding edge. A value pair of ''100% 100%'' places
        the lower right corner of the image in the lower right corner of the
        area. With a value pair of ''75% 50%'', the point 75% across
        and 50% down the image is to be placed at the point 75% across and 50%
        down the area.
        <div class="figure">
        <p><img alt="Diagram of image position within element" src="bg-pos.png">
        <p class="caption">Diagram of the meaning of ''background-position: 75% 50%''.
        </div>
      </div>

    <dt><var>&lt;length&gt;</var>

    <dd>A length value gives a fixed length as the offset. For example, with a
      value pair of ''2cm 1cm'', the upper left corner of the image
      is placed 2cm to the right and 1cm below the upper left corner of the
      background positioning area.

    <dt><dfn title="''top''!!'background-position' value">''top''</dfn>

    <dd>Equivalent to ''0%'' for the vertical position if one or two
      values are given, otherwise specifies the top edge as the origin for the
      next offset.

    <dt><dfn title="''right''!!'background-position' value">''right''</dfn>

    <dd>Equivalent to ''100%'' for the horizontal position if one or two
      values are given, otherwise specifies the right edge as the origin
      for the next offset.

    <dt><dfn title="''bottom''!!'background-position' value">''bottom''</dfn>

    <dd>Equivalent to ''100%'' for the vertical position if one or two
      values are given, otherwise specifies the bottom edge as the origin
      for the next offset.

    <dt><dfn title="''left''!!'background-position' value">''left''</dfn>

    <dd>Equivalent to ''0%'' for the horizontal position if one or two
      values are given, otherwise specifies the left edge as the origin
      for the next offset.

    <dt><dfn title="''center''!!'background-position' value">''center''</dfn>

    <dd>Equivalent to ''50%'' (''left 50%'') for the
      horizontal position if the horizontal position is not otherwise
      specified, or ''50%'' (''top 50%'') for the vertical position if
      it is.
  </dl>

  <div class="example">
    <p>The following 'background' shorthand declarations use keywords
    to set 'background-position' to the stated percentage values.</p>
    <pre>
<!-- -->body { background: url("banner.jpeg") right top }    /* 100%   0% */
<!-- -->body { background: url("banner.jpeg") top center }   /*  50%   0% */
<!-- -->body { background: url("banner.jpeg") center }       /*  50%  50% */
<!-- -->body { background: url("banner.jpeg") bottom }       /*  50% 100% */</pre>
  </div>

  <div class="example">
    <p>In the example below, the (single) image is placed in the lower-right
    corner of the viewport.
    <pre>
<!-- -->body {
<!-- -->  background-image: url("logo.png");
<!-- -->  background-attachment: fixed;
<!-- -->  background-position: 100% 100%;
<!-- -->  background-repeat: no-repeat;
<!-- -->}</pre>
  </div>

  <div class=example>
    <p>Background positions can also be relative to other corners than the
    top left. E.g., the following puts the background image 10px from the
    bottom and 3em from the right:

    <pre>background-position: right 3em bottom 10px</pre>
  </div>

  <p>See the section <a href="#layering">“Layering multiple background
  images”</a> for how 'background-position' interacts with
  other comma-separated background properties to form each background
  image layer.

<h3 id="the-background-clip">
The 'background-clip' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-clip</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;box&gt;</var> [ , <var>&lt;box&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>border-box
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>same as specified value
  </table>

  <p>Determines the <dfn id="background-painting-area">background painting
  area</dfn>. The syntax of the property is given with

  <pre class=prod><dfn><var>&lt;box&gt;</var></dfn> = border-box | padding-box | content-box</pre>

  <p>Values have the following meanings:

  <dl>
    <dt><dfn title="''border-box''!!'background-clip' value">
        ''border-box''</dfn></dt>
    <dd>The background is painted within (clipped to) the
      <span>border box.</span>
    <dt><dfn title="''padding-box''!!'background-clip' value">
        ''padding-box''</dfn></dt>
    <dd>The background is painted within (clipped to) the
      <span>padding box.</span>
    <dt><dfn title="''content-box''!!'background-clip' value">
        ''content-box''</dfn></dt>
    <dd>The background is painted within (clipped to) the
      <span>content box.</span>
  </dl>

  <p class=note>Note that the root element has a different <span>background
  painting area,</span> and thus the 'background-clip' property has no effect
  when specified on it. See <a href="#special-backgrounds"> “The
  backgrounds of special elements.”</a>

  <p class=note>Note that the background is always drawn <em>behind</em>
  the border, if any. See “Elaborate description of Stacking Contexts” in
  [[!CSS21]].

  <p>See the section <a href="#layering">“Layering multiple background
  images”</a> for how ''background-clip'' interacts with other
  comma-separated background properties to form each background image
  layer.

<h3 id="the-background-origin">
The 'background-origin' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-origin</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;box&gt;</var>
        [ , <var>&lt;box&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>padding-box
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>same as specified value
  </table>

  <p>For elements rendered as a single box, specifies the
  <dfn id="background-positioning-area">background positioning area</dfn>.
  For elements rendered as multiple boxes (e.g., inline boxes on several
  lines, boxes on several pages) specifies which boxes 'box-decoration-break'
  operates on to determine the background positioning area(s).

  <dl>
    <dt><dfn title="''padding-box''!!'background-origin' value">
        ''padding-box''</dfn>
    <dd>The position is relative to the padding box. (For single boxes
      ''0 0'' is the upper left corner of the padding edge,
      ''100% 100%'' is the lower right corner.)
    <dt><dfn title="''border-box''!!'background-origin' value">
        ''border-box''</dfn>
    <dd>The position is relative to the border box.
    <dt><dfn>''content-box''</dfn>
    <dd>The position is relative to the content box.
  </dl>

  <p>If the 'background-attachment' value for this image is
  ''fixed'', then this property has no effect: in this case the
  background positioning area is the
  <a href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">initial containing block</a> [[!CSS21]].

  <p class="note">Note that if 'background-clip' is ''padding-box'',
  'background-origin' is ''border-box'', 'background-position' is
  ''top left'' (the initial value), and the element has a
  non-zero border, then the top and left of the background image will be
  clipped.

  <p>See the section <a href="#layering">“Layering multiple background
  images”</a> for how ''background-origin'' interacts with other
  comma-separated background properties to form each background image
  layer.


<h3 id="the-background-size">
The 'background-size' property</h3>

  <table class="propdef">
    <tr>
      <th>Name:
      <td><dfn>background-size</dfn>
    <tr>
      <th>Value:
      <td><var>&lt;bg-size&gt;</var> [ , <var>&lt;bg-size&gt;</var> ]*
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>see text
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed value:
      <td>for &lt;length&gt; the absolute value, otherwise as specified
  </table>

  <p>Specifies the size of the background images. Where

  <pre class=prod><dfn><var>&lt;bg-size&gt;</var></dfn> = [ <var>&lt;length&gt;</var> | <var>&lt;percentage&gt;</var> | auto ]{1,2} | cover | contain</pre>

  <p>Values have the following meanings:

  <dl>
    <dt><dfn>''contain''</dfn></dt>
    <dd>Scale the image, while preserving its intrinsic aspect ratio
      (if any), to the largest size such that both its width and its
      height can fit inside the <span class=index>background
      positioning area.</span></dd>

    <dt><dfn>''cover''</dfn></dt>
    <dd>Scale the image, while preserving its intrinsic aspect ratio
      (if any), to the smallest size such that both its width and its
      height can completely cover the background positioning area.</dd>

    <dt>[ <var>&lt;length&gt;</var> | <var>&lt;percentage&gt;</var>
    | <dfn title="''auto''!!'background-size' value">auto</dfn> ]{1,2}</dt>
    <dd>
      <p>The first value gives the width of the corresponding image,
      the second value its height. If only one value is given the second
      is assumed to be ''auto''.

      <p>A percentage is relative to the background positioning area.

      <p>An ''auto'' value for one dimension is resolved by using the
      image's intrinsic ratio and the size of the other dimension, or
      failing that, using the image's intrinsic size, or failing that,
      treating it as 100%.

      <p>If both values are ''auto'' then the intrinsic width and/or
      height of the image should be used, if any<!--, the missing dimension
      (if any) behaving as ''auto'' as described above-->. If the image has
      neither an intrinsic width nor an intrinsic height, its size is
      determined as for ''contain''.

      <p>Negative values are not allowed.
    </dd>
  </dl>

  <div class="example">
    <p>Here are some examples. The first example stretches the background
    image independently in both dimensions to completely cover the content
    area:

    <pre>
<!-- -->div {
<!-- -->    background-image: url(plasma.png);
<!-- -->    background-repeat: no-repeat;
<!-- -->    background-size: 100% 100%;
<!-- -->    background-origin: content-box }</pre>

    <p>The second example stretches the image so that exactly two copies
    fit horizontally. The aspect ratio is preserved:

    <pre>
<!-- -->p {
<!-- -->    background-image: url(tubes.png);
<!-- -->    background-size: 50% auto;
<!-- -->    background-origin: border-box }</pre>

<!-- --><p>This example forces the background image to be 15 by 15 pixels:

    <pre>
<!-- -->para {
<!-- -->    background-size: 15px 15px;
<!-- -->    background-image: url(tile.png) }</pre>

    <p>This example uses the image's intrinsic size. Note that this is the
    only possible behavior in CSS level&nbsp;1 and 2.

    <pre>
<!-- -->body {
<!-- -->    background-size: auto;            /* default */
<!-- -->    background-image: url(flower.png) }</pre>

    <p>The following example rounds the height of the image to 33.3%, up
    from the specified value of 30%. At 30%, three images would fit
    entirely and a fourth only partially. After rounding, three images
    fit. The width of the image is 20% of the background area width and is
    not rounded.

    <pre>
<!-- -->p {
<!-- -->    background-image: url(chain.png);
<!-- -->    background-repeat: no-repeat round;
<!-- -->    background-size: 20% 30% }</pre>
  </div>

  <p>If 'background-repeat' is ''round'' for one (or both) dimensions,
  there is a second step. The UA must scale the image in that dimension
  (or both dimensions) so that it fits a whole number of times in
  the background positioning area. In the case of the width (height is
  analogous):

  <blockquote>
    <p>If <var>X</var> &ne; 0 is the width of the image after step one
    and <var>W</var> is the width of the background positioning area,
    then the rounded width
    <var>X'</var> = <var>W</var> / round(<var>W</var> / <var>X</var>)
    where round() is a function that returns the nearest natural number