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<title>PIE Documentation: Supported CSS3 Features</title>
<h1>Supported CSS3 Features</h1>
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<p>The following sections describe in detail the exact levels of support PIE has for certain CSS3 properties
and value types.</p>
<li><a href="#border-radius">border-radius</a></li>
<li><a href="#box-shadow">box-shadow</a></li>
<li><a href="#border-image">border-image</a></li>
<li><a href="#pie-background">CSS3 Backgrounds (-pie-background)</a></li>
<li><a href="#gradients">Gradients</a></li>
<li><a href="#rgba">RGBA Color Values</a></li>
<li><a href="#custom">PIE Custom Properties</a>
<li><a href="#pie-watch-ancestors">-pie-watch-ancestors</a></li>
<li><a href="#pie-png-fix">PNG alpha transparency and -pie-png-fix</a></li>
<li><a href="#pie-lazy-init">Lazy Initialization (-pie-lazy-init)</a></li>
<li><a href="#pie-poll">Layout polling (-pie-poll)</a></li>
<li><a href="#pie-track-hover">:hover tracking (-pie-track-hover)</a></li>
<li><a href="#pie-track-active">:active tracking (-pie-track-active)</a></li>
<h2 id="border-radius">border-radius</h2>
<p>PIE fully supports the border-radius property as defined in the CSS3
Backgrounds and Borders module specification for IE 6-8. IE 9 and up support
border-radius natively.</p>
<pre class="long">border-radius: [ &lt;length&gt; | &lt;percentage&gt; ]{1,4} [ / [ &lt;length&gt; | &lt;percentage&gt; ]{1,4} ]?</pre>
<p>Only the shorthand version is supported; the longhand
border-top-left-radius etc. properties are not. The shorthand syntax does support different radii per corner, though:</p>
<p><code>border-radius: 5px 10px 15px 20px;</code> (top-left, top-right, bottom-right, bottom-left).</p>
<p>The rounded corners are applied to the element's background area
(including solid background colors, background images, and background
gradients), the element's border, and the box-shadow if specified.</p>
<p>Both the standard border-radius property name as well as a custom
prefixed -pie-border-radius property name are recognized; if both are
present then the prefixed value will take precedence. It is recommended
to only use the standard unprefixed property when possible.</p>
<h2 id="box-shadow">box-shadow</h2>
<p>PIE supports the <a href="">box-shadow property syntax</a>
as defined in the CSS3 Backgrounds &amp; Borders module, for IE 6-8. IE 9 and up support
box-shadow natively so PIE does nothing.</p>
<pre class="long">box-shadow: none | &lt;shadow&gt; [,&lt;shadow&gt;]*
where &lt;shadow&gt; = inset? &amp;&amp; [ &lt;offset-x&gt; &lt;offset-y&gt; &lt;blur-radius&gt;? &lt;spread-radius&gt;? &amp;&amp; &lt;color&gt;? ]</pre>
<p>Both the standard box-shadow property name as well as a custom
prefixed -pie-box-shadow property name are recognized; if both are
present the prefixed value will take precedence. It is recommended to
use the non-prefixed property when possible.</p>
<p>When used in conjunction with border-radius, the shape of the shadow
matches the shape of the rounded border box.</p>
<p>The shadow shape rendered by PIE is, unlike other browsers,
<a href="">opaque in the area behind the element's background</a>.
This means that if your element has a transparent or semi-transparent background, the opaque shadow will
show through. To avoid this you must either give the element a non-transparent background, or remove the
<p>PIE does not currently support the 'inset' keyword, but support is
planned in a future version (see <a href="">issue #3</a>.</p>
<p>Notes on other browsers:</p>
<p>See the compatibility chart at the bottom of
<a href=""></a></p>
<h2 id="border-image">border-image</h2>
<p>PIE adds support for the border-image property to IE 6-9. This property allows
you to specify an image which gets divided into nine squares which are then drawn as
the corners, sides, and center of the target element.</p>
<p>There are currently a few limitations of PIE's implementation, including:</p>
<li>It only supports the 'stretch' scheme in IE 6-8. (The other schemes ('repeat' and 'round') are
fully supported in IE 9.)</li>
<li>It doesn't support the <a href="">outset parameter</a>
described in the Backgrounds &amp; Borders module spec, though other browsers don't seem to support
that yet either.</li>
<li>It requires the 'fill' keyword to be present for the center area to be filled in. This is correct
behavior according to the spec but other browsers don't require it and some even fail if 'fill' is
present, so it's a bit tricky making it work consistently across browsers.</li>
<li>It seems there are on rare occasion rounding errors which cause 1px gaps between slices of the
image. I've seen these gaps occur in other browsers too, though.</li>
<h2 id="pie-background">CSS3 Backgrounds (-pie-background)</h2>
<p>PIE supports CSS3 multiple background images, linear gradients as
background images, and some of the new CSS3 background aspects such as
background origin and clip. Unfortunately, to get access to these
post-CSS2 values, we have to put them in a property other than the
standard 'background' property, because IE will attempt to parse the
value internally and not allow us access to the original value string.
Therefore we use a custom -pie-background property for holding these
<p>(Note that IE9 and up support multiple background images natively, so PIE only
takes over in IE 6-8. It also takes over in IE 9 when there is a gradient present in the
-pie-background -- see the section on <a href="#gradients">gradients</a> below.)</p>
<p>Only the single -pie-background shorthand value is recognized; longhand values (e.g.
-pie-background-origin) are ignored.</p>
<p>For backward-compatibility with browsers which do not support CSS3
backgrounds, be sure to include appropriate fallbacks. For example:</p>
<pre class="long"><code>#myElement {
background: url(bg-image.png) no-repeat #CCC; /*non-CSS3 browsers will use this*/
background: url(bg-image.png) no-repeat, -webkit-gradient(linear, 0 0, 0 100%, from(#CCC) to(#EEE)); /*old webkit*/
background: url(bg-image.png) no-repeat, -webkit-linear-gradient(#CCC, #EEE); /*new webkit*/
background: url(bg-image.png) no-repeat, -moz-linear-gradient(#CCC, #EEE); /*gecko*/
background: url(bg-image.png) no-repeat, -ms-linear-gradient(#CCC, #EEE); /*IE10 preview*/
background: url(bg-image.png) no-repeat, -o-linear-gradient(#CCC, #EEE); /*opera 11.10+*/
background: url(bg-image.png) no-repeat, linear-gradient(#CCC, #EEE); /*future CSS3 browsers*/
-pie-background: url(bg-image.png) no-repeat, linear-gradient(#CCC, #EEE); /*PIE*/
<p>While the PIE parser will allow them, the following aspects of the
background shorthand will currently be ignored when rendering in IE 6-8:</p>
<li>background-attachment (will always use 'scroll' even if 'fixed'
or 'local' are specified)</li>
<li>background-size (will always use the image's intrinsic size)</li>
<li>background-repeat values of 'space' or 'round' (the other repeat
values are supported)</li>
<li>background-origin (will always use 'padding-box')</li>
<li>background-position values with more than 2 parts</li>
<p>Support for these items will be added in future versions as possible.</p>
<p>Note that PNG background images specified using <code>-pie-background</code> will be
rendered with correct alpha channel transparency in IE6. See the section below regarding
<a href="#pie-png-fix">PNG alpha transparency</a> for more information.</p>
<p>Notes on other browsers:</p>
<p>See <a href=""></a></p>
<h2 id="gradients">Gradients</h2>
<p>PIE currently supports <code>linear-gradient</code> image values when used in the
-pie-background property, for IE 6-9. Uses of linear-gradient in any contexts other than the -pie-background
are not supported. The supported syntax matches that of the current
<a href="">CSS3 Image Values module</a> draft.</p>
<pre class="long">linear-gradient([&lt;bg-position&gt; || &lt;angle&gt;,]? &lt;color-stop&gt;, &lt;color-stop&gt;[, &lt;color-stop&gt;]*);</pre>
<p>PIE's linear-gradient support in IE 6-8 has a few limitations (all of these work
properly in IE 9):</p>
<li>Currently all color stops are rendered fully opaque, even if
specifying an rgba color value. This is due to a limitation in VML's
linear gradient syntax which does not allow setting opacity for
individual color stops. (See <a href="">issue #7</a>)</li>
<li>Gradients containing color-stops which lie outside the bounding area
of the element are not currently supported, due to limitations in VML's
gradient rendering.</li>
<li>The background-size/origin/repeat/position parameters are not supported
for gradients, so you cannot do things like
<a href="">gradient pattern effects</a>.</li>
<p>Radial gradients are not supported at this time; this feature is planned
for a future release (see <a href="">issue #2</a>)
but it may turn out to be impossible to implement in IE 6-8 due to VML's strange radial gradient behavior.</p>
<p>Notes on other browsers:</p>
<p>Most other browsers require a vendor prefix on the linear-gradient name, e.g. -moz-linear-gradient for Firefox
(supported in version 3.6+ only), -webkit-linear-gradient for recent WebKit browsers, -o-linear-gradient for Opera
(supported in version 11.10+ only), and -ms-linear-gradient for IE10 (currently only in pre-release).</p>
<p>In addition, WebKit browsers older than Chrome 10 and Safari 5.1 require a gradient syntax which is
drastically different than that of the CSS3 spec. See the
<a href="">Safari documentation</a> for their syntax</p>
<p>Adding in PIE's required -pie-background property, you will need a set of styles similar to the following
to get consistent linear gradient backgrounds across browsers:</p>
<pre class="long"><code>#myElement {
background: #CCC; /*fallback for non-CSS3 browsers*/
background: -webkit-gradient(linear, 0 0, 0 100%, from(#CCC) to(#EEE)); /*old webkit*/
background: -webkit-linear-gradient(#CCC, #EEE); /*new webkit*/
background: -moz-linear-gradient(#CCC, #EEE); /*gecko*/
background: -ms-linear-gradient(#CCC, #EEE); /*IE10*/
background: -o-linear-gradient(#CCC, #EEE); /*opera 11.10+*/
background: linear-gradient(#CCC, #EEE); /*future CSS3 browsers*/
-pie-background: linear-gradient(#CCC, #EEE); /*PIE*/
behavior: url(;
<p>For more detailed information on the current state of linear-gradient across browsers, see this
<a href="">article by John Allsopp</a>.</p>
<h2 id="rgba">RGBA Color Values</h2>
<p>PIE parses RGBA color values wherever they are allowed. However it
is only able to successfully render their opacity value in a few
contexts. In all other contexts they will be rendered with the correct
RGB color, but fully opaque. Here are the supported contexts in which
the opacity will be rendered correctly:</p>
<li>The solid background-color as specified in the -pie-background property.</li>
<li>The color value of box-shadow, if the shadow has no blur.</li>
<li>Color stops in linear-gradient, for IE 9 only (will be rendered fully opaque in IE 6-8)</li>
<h2 id="custom">PIE custom properties</h2>
<h3 id="pie-watch-ancestors">-pie-watch-ancestors</h3>
<p>PIE automatically listens for any attribute or style property
changes on the element to which the behavior is applied. This means
that if you have scripting which modifies any of the recognized CSS3
properties on the fly, those changes will automatically be picked up
and the rendering will be updated to match. For example:</p>
<pre><code>/* JS: */
myElement.onclick = function() { = '20px';
/* CSS: */
#myElement {
behavior: url(;
border-radius: 10px;
<p>Assuming myElement has the behavior attached to it, the
above code will work as expected without any extra effort from the
author of the script or CSS. This seamlessness is a big part of why PIE
is so easy to use.</p>
<p>But common best-practices in scripting dictate that instead of
setting styles directly with, scripts should only
add/remove elements' class names, letting the actual styles
corresponding to those class names be maintained in the CSS. So
reworking the above example:</p>
<pre><code>/* JS: */
myElement.onclick = function() {
this.className += ' poked';
/* CSS: */
#myElement {
behavior: url(;
border-radius: 10px;
#myElement.poked {
border-radius: 20px;
<p>Again, since the className is being changed on the element to which
the behavior is applied, PIE will automatically be notified of the
change and update the border-radius rendering to match the new value.</p>
<p>However, what if the className is changed not on the element itself
but on one of its ancestors?</p>
<pre><code>/* JS: */
myElement.onclick = function() {
this.parentNode.className += ' poked';
/* CSS: */
#myElement {
behavior: url(;
border-radius: 10px;
.poked #myElement {
border-radius: 20px;
<p>This is a very common pattern which allows a lot of flexibility.
However, in this case, PIE will not be automatically notified of the
className change. To be notified, it will also have to add a listener
to the ancestor element. We could brute-force this by automatically
adding propertychange listeners to all the ancestors of every
PIE-targeted element, but that would be bad for performance and memory
usage. So instead, we have introduced a custom CSS property which
allows authors to tell PIE that certain ancestors should be watched:</p>
<pre><code>/* JS: */
myElement.onclick = function() {
this.parentNode.className += ' poked';
/* CSS: */
#myElement {
behavior: url(;
border-radius: 10px;
<strong>-pie-watch-ancestors: 1;</strong>
.poked #myElement {
border-radius: 20px;
<p>This tells PIE that it should watch for changes on ancestors one
level up from the element. It will attach the propertychange listener
to the element's parent and therefore be notified when the parent's
className gets changed, and update the rendering correctly.
<h3 id="pie-png-fix">PNG alpha transparency and -pie-png-fix</h3>
<p>A nice side-effect of PIE's use of VML for rendering is that it causes PNG images
with alpha channel transparency to be correctly displayed in IE6 when they are rendered
by PIE's engine. This includes:</p>
<li>Background images specified using the <a href="#pie-background">-pie-background</a> property</li>
<li>Background images specified using the standard <code>background-image</code> style, when used
in conjunction with other CSS3 properties that trigger re-rendering of the background
(border-radius, border-image)</li>
<li>&lt;img> elements that have border-radius applied</li>
<p>Sometimes you might want the benefit of the fixed PNG transparency, on elements that do not meet
the criteria above. In that case, you can add the custom property <code>-pie-png-fix: true;</code> to
force re-rendering of the background-image or &lt;img>. (The behavior must also be
attached to the element.)</p>
<h3 id="pie-lazy-init">Lazy Initialization (-pie-lazy-init)</h3>
<p>While PIE has been optimized for speed, there is still a small cost in rendering performance for each element
it is applied to. When you have dozens or hundreds of elements on your page with CSS3 styles applied, this
can add up to a noticeable rendering delay.</p>
<p>When you have that many elements on one page, chances are that only a small number of them are visible
in the browser viewport initially, as viewing the rest would require scrolling. PIE allows an optional
optimization for this case: if you apply the custom <code>-pie-lazy-init:true;</code> property to elements
PIE will delay the initialization of their CSS3 rendering until they are scrolled into the viewport. This
keeps the initial page load snappy without severely limiting the number of elements you can render.</p>
<h3 id="pie-poll">Layout Polling (-pie-poll)</h3>
<p>In general PIE is quite good at detecting changes to the size and position of the elements to which it
is attached and automatically adjusting its rendering to match. It does this by listening to the IE-specific
<code>onmove</code> and <code>onresize</code> events for each target element. In the majority of cases this
works seamlessly; in rare cases, however, IE does not fire these events when it should, and PIE gets out of sync.</p>
<p>To help users get around these cases, PIE has a second method for tracking size and position changes: polling.
When polling is enabled for an element, PIE will manually query that element's layout several times a second, and
if the layout has changed then it will adjust the rendering.</p>
<p>Polling is enabled by default for all elements in IE 8 and 9 (as those version is particularly bad about not firing
the events) and disabled in IE 6 and 7. Users can override these defaults
to force polling on or off for individual elements by setting a custom CSS property: just specify
<code>-pie-poll:true;</code> to force polling on for an element, or <code>-pie-poll:false;</code> to disable it.</p>
<h3 id="pie-track-hover">Hover tracking (-pie-track-hover)</h3>
<p>By default, PIE automatically listens for mouseover/out events and applies any matching <code>:hover</code>
styles if supported by the browser. In addition, it adds a special <code>pie_hover</code> class to the element
which you can use to apply hover styles in versions of IE that don't support it.</p>
<p>If you want to disable this, you can do so by setting the custom CSS style <code>-pie-track-hover:false;</code>
for the element. This might be useful if you notice sluggishness on your page when hovering and you don't need
hover styles.</p>
<h3 id="pie-track-active">Active tracking (-pie-track-active)</h3>
<p>By default, PIE automatically listens for mousedown/up events and applies any matching <code>:active</code>
styles if supported by the browser. In addition, it adds a special <code>pie_active</code> class to the element
which you can use to apply active styles in versions of IE that don't support it.</p>
<p>If you want to disable this, you can do so by setting the custom CSS style <code>-pie-track-active:false;</code>
for the element. This is useful in some situations where the addition of the pie_active class has unwanted
side effects, for instance breaking the behavior of <a href="">scrollbars in IE7</a>.</p>
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