PIE Documentation: Supported CSS3 Features

# Supported CSS3 Features

The following sections describe in detail the exact levels of support PIE has for certain CSS3 properties and value types.

- border-radius
- box-shadow
- border-image
- CSS3 Backgrounds (-pie-background)
- Gradients
- RGBA Color Values
- PIE Custom Properties
  - -pie-watch-ancestors
  - PNG alpha transparency and -pie-png-fix
  - Lazy Initialization (-pie-lazy-init)
  - Layout polling (-pie-poll)

## border-radius

PIE fully supports the border-radius property as defined in the CSS3 Backgrounds and Borders module specification.

    border-radius: [ <length> | <percentage> ]{1,4} [ / [ <length> | <percentage> ]{1,4} ]?

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:

`border-radius: 5px 10px 15px 20px;` (top-left, top-right, bottom-right, bottom-left).

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.

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 (in addition, of course, to -moz- and -webkit- prefixed properties as necessary), since the CSS3 spec for this property seems fairly solidified, and because there are no known incompatibilities between PIE's implementation and the standard.

Notes on other browsers:

Elliptical radii (the optional set of values after the slash) are only supported in Firefox versions 3.5 and up. As of the current release (3.6), percentage values for vertical radii are incorrectly calculated relative to the width of the box whereas the CSS3 spec says they should be relative to the height. PIE handles this correctly.

WebKit deviates significantly from the CSS3 spec: the shorthand property cannot be used to specify different radii per corner, so you must use the separate longhand properties if you want to do that. It also does not support percentage values at all. (TODO: check this in WebKit nightlies and recent Chrome, I think these may now match the spec and drop the prefix.)

## box-shadow

PIE supports the box-shadow property syntax as currently defined in the CSS3 Backgrounds & Borders module.

    box-shadow: none | <shadow> [,<shadow>]*
        where <shadow> = inset? && [ <offset-x> <offset-y> <blur-radius>? <spread-radius>? && <color>? ]

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, since there are no known incompatibilities between PIE's implementation and others which use the non-prefixed property, such as Opera.

When used in conjunction with border-radius, the shape of the shadow matches the shape of the rounded border box.

When using the blur radius parameter, the strength and size of the blur as rendered by PIE should match that of other browsers almost identically, however you may notice some slight differences due to rounding, particularly when the blur is very small (this should not be more than a one-pixel difference).

PIE does not currently support the 'inset' keyword, but support is planned in a future version (see issue #3.

Notes on other browsers:

See the compatibility chart at the bottom of https://developer.mozilla.org/En/CSS/-moz-box-shadow

## border-image

PIE has preliminary support for the border-image property. 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. The sides and center will stretch to match the size of the element.

There are currently several limitations of PIE's implementation, including:

1. It only supports the 'stretch' scheme. I'd like to support the others in the future but performance is a big concern as it seems it will require creating a separate VML shape for each tile.
2. It doesn't support the outset parameter described in the Backgrounds & Borders module spec, though other browsers don't seem to support that yet either.
3. It doesn't support the width override parameter to the border-image shorthand; it will only use the element's border-width for determining the slice widths (again, support for this in other browsers is spotty too).
4. Related to #3, if the element's `border-style` is 'none' it will treat that as `border-width:0`.
5. 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.
6. It doesn't hide the element's normal border when border-image is specified. In conjunction with #4 above, this means you have to set the border-style to 'solid' (or something other than 'none') and set the border-color to transparent.
7. 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.

These issues will be addressed in a future release.

## CSS3 Backgrounds (-pie-background)

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 values.

Only the single -pie-background shorthand value is recognized; longhand values (e.g. -pie-background-origin) are ignored.

For backward-compatibility with browsers which do not support CSS3 backgrounds, be sure to include appropriate fallbacks. For example:

    #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*/
    }

While the PIE parser will allow them, the following aspects of the background shorthand will currently be ignored when rendering:

- background-attachment (will always use 'scroll' even if 'fixed' or 'local' are specified)
- background-size (will always use the image's intrinsic size)
- background-repeat values of 'space' or 'round' (the other repeat values are supported)
- background-origin (will always use 'padding-box')
- background-position values with more than 2 parts

Support for these items will be added in future versions as possible.

Note that PNG background images specified using `-pie-background` will be rendered with correct alpha channel transparency in IE6. See the section below regarding PNG alpha transparency for more information.

Notes on other browsers:

Firefox supports multiple backgrounds as of version 3.6. Safari supports them as of version 1.3. See https://developer.mozilla.org/en/CSS/Multiple_backgrounds

## Gradients

PIE currently supports `linear-gradient` image values when used in the -pie-background property. Uses in any contexts other than the background are not supported. The supported syntax matches that of the current CSS3 Image Values module draft.

    linear-gradient([<bg-position> || <angle>,]? <color-stop>, <color-stop>[, <color-stop>]*);

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 issue #7)

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.

Radial gradients are not supported at this time; this feature is planned for a future release (see issue #2) but it may turn out to be impossible to implement due to VML's strange radial gradient behavior.

Notes on other browsers:

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).

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 Safari documentation for their syntax

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:

    #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(PIE.htc);
    }

For more detailed information on the current state of linear-gradient across browsers, see this article by John Allsopp.

## RGBA Color Values

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:

- The solid background-color as specified in the -pie-background property.
- The color value of box-shadow, if the shadow has no blur.

## PIE custom properties

### -pie-watch-ancestors

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:

    /* JS: */
    myElement.onclick = function() {
        this.style.borderRadius = '20px';
    };

    /* CSS: */
    #myElement {
        behavior: url(PIE.htc);
        border-radius: 10px;
    }

Assuming myElement has the PIE.htc 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.

But common best-practices in scripting dictate that instead of setting styles directly with element.style.foo, 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:

    /* JS: */
    myElement.onclick = function() {
        this.className += ' poked';
    }

    /* CSS: */
    #myElement {
        behavior: url(PIE.htc);
        border-radius: 10px;
    }
    #myElement.poked {
        border-radius: 20px;
    }

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.

However, what if the className is changed not on the element itself but on one of its ancestors?

    /* JS: */
    myElement.onclick = function() {
        this.parentNode.className += ' poked';
    }

    /* CSS: */
    #myElement {
        behavior: url(PIE.htc);
        border-radius: 10px;
    }
    .poked #myElement {
        border-radius: 20px;
    }

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:

    /* JS: */
    myElement.onclick = function() {
        this.parentNode.className += ' poked';
    }

    /* CSS: */
    #myElement {
        behavior: url(PIE.htc);
        border-radius: 10px;
        -pie-watch-ancestors: 1;
    }
    .poked #myElement {
        border-radius: 20px;
    }

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.

### PNG alpha transparency and -pie-png-fix

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:

- Background images specified using the -pie-background property
- Background images specified using the standard `background-image` style, when used in conjunction with other CSS3 properties that trigger re-rendering of the background (border-radius, border-image)
- <img> elements that have border-radius applied

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 `-pie-png-fix: true;` to force re-rendering of the background-image or <img>. (The PIE.htc behavior must also be attached to the element.)

### Lazy Initialization (-pie-lazy-init)

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.

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 `-pie-lazy-init:true;` 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.

### Layout Polling (-pie-poll)

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 `onmove` and `onresize` 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.

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.

Polling is enabled by default for all elements in IE8 (as that 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 `-pie-poll:true;` to force polling on for an element, or `-pie-poll:false;` to disable it.