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[css2] Issue 149
http://wiki.csswg.org/spec/css2.1#issue-142 --HG-- extra : convert_revision : svn%3A73dc7c4b-06e6-40f3-b4f7-9ed1dbc14bfc/trunk%403078
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‎css2/syndata.src‎

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<html lang="en">
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<!-- $Id: syndata.src,v 2.180 2010-09-29 15:52:58 bbos Exp $ -->
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<!-- $Id: syndata.src,v 2.181 2010-10-28 18:20:30 bbos Exp $ -->
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<head>
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<title>Syntax and basic data types</title>
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<meta name="editor" lang="tr" content="Tantek &Ccedil;elik">
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<h3><a name="length-units">Lengths</a></h3>
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<p>Lengths refer to horizontal or vertical measurements.</p>
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<p>Lengths refer to distance measurements.</p>
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<p> The format of a length value (denoted by <span class="index-def"
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title="&lt;length&gt;::definition of"><a name="value-def-length"
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negative length values, the declaration is ignored.
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</p>
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<p>In cases where the <a href="cascade.html#usedValue">used</a>
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length cannot be supported, user agents must approximate it in the
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<a href="cascade.html#actual-value">actual value.</a>
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<p><a name="absrel-units">There are two types of length units:
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relative and absolute.</a> <span class="index-def" title="relative
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units"><em>Relative length</em></span> units specify a length relative
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to another length property. Style sheets that use relative units will
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more easily scale from one medium to another (e.g., from a computer
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display to a laser printer).
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to another length property. Style sheets that use relative units
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can more easily scale from one output environment
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to another.
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</p>
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<p>Relative units are:</p>
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<ul>
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<li><strong>em</strong>: the <span class="propinst-font-size">'font-size'</span> of the relevant font</li>
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<li><strong>ex</strong>: the <span class="descinst">'x-height'</span> of the relevant font</li>
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<li><strong>px</strong>: pixels, relative to the viewing device</li>
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<li></li>
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</ul>
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<div class="example">
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<pre><code class="css">
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h1 { margin: 0.5em } /* em */
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h1 { margin: 1ex } /* ex */
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p { font-size: 12px } /* px */
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</code></pre>
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</div>
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document tree</a> (e.g., "HTML" in HTML), 'em' and 'ex' refer to
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the property's <a href="about.html#initial-value">initial value</a>.
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</p>
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<p><a name="pixel-units">Pixel units</a> are relative to the
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resolution of the viewing device, i.e., most often a computer
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display. If the pixel density of the output device is very different
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from that of a typical computer display, the user agent should rescale
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pixel values. It is recommended that the pixel unit refer to the whole
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number of device pixels that best approximates the reference pixel. It
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is recommended that the <span class="index-def"
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title="reference pixel|pixel"><em>reference pixel</em></span> be the
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visual angle of one pixel on a device with a pixel density of 96dpi
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and a distance from the reader of an arm's length. For a nominal arm's
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length of 28 inches, the visual angle is therefore about 0.0213
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degrees.
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</p>
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<p>For reading at arm's length, 1px thus corresponds to about 0.26&nbsp;mm
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(1/96&nbsp;inch). When printed on a laser printer, meant for reading at a
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little less than arm's length (55&nbsp;cm, 21&nbsp;inches), 1px is about
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0.20&nbsp;mm. On a 300 dots-per-inch (dpi) printer, that may be
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rounded up to 3 dots (0.25&nbsp;mm); on a 600&nbsp;dpi printer, it can
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be rounded to 5 dots.
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</p>
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<p>The two images below illustrate the effect of viewing distance on
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the size of a pixel and the effect of a device's resolution. In the
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first image, a reading distance of 71&nbsp;cm (28&nbsp;inches) results
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in a px of 0.26&nbsp;mm, while a reading distance of 3.5&nbsp;m
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(12&nbsp;feet) requires a px of 1.3&nbsp;mm.
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</p>
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<div class="figure">
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<p><img src="images/pixel1.png" alt="Showing that pixels must become
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larger if the viewing distance increases"> </p>
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</div>
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<p>In the second image, an
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area of 1px by 1px is covered by a single dot in a low-resolution
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device (a computer screen), while the same area is covered by 16 dots
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in a higher resolution device (such as a 400&nbsp;dpi laser printer).
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</p>
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<div class="figure">
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<p><img
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src="images/pixel2.png" alt="Showing that more device pixels (dots)
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are needed to cover a 1px by 1px area on a high-resolution device than
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on a low-res one"></p>
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</div>
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<p>Child elements do not inherit the relative values specified for
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their parent; they inherit the <a
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</div>
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<p><span class="index-def" title="absolute length"><em> Absolute
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length</em></span> units are only useful when the physical properties
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of the output medium are known. The absolute units are:
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length</em></span>
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units are fixed in relation to each other.
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They are mainly useful
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when the output
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environment is known. The absolute units consist of the physical units
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(in, cm, mm, pt, pc) and the px unit:
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</p>
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<ul>
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<li><strong>in</strong>: inches &mdash; 1 inch is equal to 2.54 centimeters.</li>
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<li><strong>in</strong>: inches &mdash; 1in is equal
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to 2.54cm.</li>
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<li><strong>cm</strong>: centimeters</li>
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<li><strong>mm</strong>: millimeters</li>
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<li><strong>pt</strong>: points &mdash; the points used by CSS&nbsp;2.1 are equal to 1/72nd of an inch. </li>
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<li><strong>pc</strong>: picas &mdash; 1 pica is equal to 12 points.</li>
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<li><strong>pt</strong>: points &mdash; the points used by CSS are equal to
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1/72nd of 1in. </li>
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<li><strong>pc</strong>: picas &mdash; 1pc is equal
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to 12pt.</li>
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<li><strong>px</strong>: pixel units &mdash; 1px is equal to 0.75pt.</li>
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</ul>
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<p>For a CSS device, these dimensions are either anchored (i) by
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relating the physical units to their physical measurements, or
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(ii) by relating the pixel unit to the <i>reference pixel</i>.
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For print media and similar high-resolution devices, the anchor unit
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should be one of the standard physical units (inches, centimeters, etc).
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For lower-resolution devices, and devices with unusual viewing distances,
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it is recommended instead that the anchor unit be the pixel unit. For
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such devices it is recommended that the pixel unit refer to the whole
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number of device pixels that best approximates the reference pixel.
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<p class="note">Note that if the anchor unit is the pixel unit,
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the physical units might not match their physical measurements.
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Alternatively if the anchor unit is a physical unit, the pixel
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unit might not map to a whole number of device pixels.</p>
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<p class="note">Note that this definition of the pixel unit and
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the physical units differs from previous versions of CSS. In
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particular, in previous versions of CSS the pixel unit and the
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physical units were not related by a fixed ratio: the physical
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units were always tied to their physical measurements while the
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pixel unit would vary to most closely match the reference pixel.
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(This change was made because too much existing content relies
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on the assumption of 96dpi, and breaking that assumption breaks
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the content.)
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<p>The <span class="index-def" title="reference pixel|pixel"><em>reference pixel</em></span> is the
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visual angle of one pixel on a device with a pixel density of 96dpi
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and a distance from the reader of an arm's length. For a nominal arm's
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length of 28 inches, the visual angle is therefore about 0.0213
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degrees. For reading at arm's length, 1px thus corresponds to about
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0.26&nbsp;mm (1/96&nbsp;inch).
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</p>
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<p>The image below illustrates the effect of viewing distance on
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the size of a reference pixel: a reading distance of 71&nbsp;cm
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(28&nbsp;inches) results in a reference pixel of 0.26&nbsp;mm,
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while a reading distance of 3.5&nbsp;m (12&nbsp;feet) results in
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a reference pixel of 1.3&nbsp;mm.
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</p>
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<div class="figure">
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<p><img src="images/pixel1.png" alt="Showing that pixels must become
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larger if the viewing distance increases"></p>
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</div>
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<p>This second image illustrates the effect of a device's resolution
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on the pixel unit: an area of 1px by 1px is covered by a single dot
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in a low-resolution device (e.g. a typical computer display), while
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the same area is covered by 16 dots in a higher resolution device
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(such as a printer).
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</p>
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<div class="figure">
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<p><img src="images/pixel2.png" alt="Showing that more device pixels (dots)
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are needed to cover a 1px by 1px area on a high-resolution device than
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on a low-res one"></p>
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</div>
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<div class="example">
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<pre><code class="css">
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h1 { margin: 0.5in } /* inches */
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h2 { line-height: 3cm } /* centimeters */
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h3 { word-spacing: 4mm } /* millimeters */
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h4 { font-size: 12pt } /* points */
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h4 { font-size: 1pc } /* picas */
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p { font-size: 12px } /* px */
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</code></pre>
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</div>
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<p>In cases where the <a href="cascade.html#usedValue">used</a> length cannot be supported, user agents must approximate it in the <a
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href="cascade.html#actual-value">actual value.</a>
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</p>
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<h3> <a name="percentage-units">Percentages</a></h3>
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<p> The format of a percentage value (denoted by <span

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