Why does a filter gradient on a pseudo element not work in IE8?

Question

I want to create buttons like these:

In modern browsers the effect is created using inset box-shadow and filters.
For IE8 - pseudo-elements are chosen.

Answer 1

The question is "Why don't filters work on pseudo elements in IE8?" The following is as close to a definitive answer as I can muster. It comes from the information on this page.

The gradient filter is a "procedural surface" (along with alphaimageloader). A procedural surface is defined so:

Procedural surfaces are colored surfaces that display between the content of an object and the object's background.

Read that carefully. It is essentially another "layer" you might say between the content of an object and that object's background. Do you see the answer to the question? What is created by :before and :after... Yes! Content. Specifically as MSDN notes:

The ::before and ::after pseudo-elements specify the location of content before and after an element in the document tree. The content attribute, in conjunction with these pseudo-elements, specifies what is inserted.

The generated content interacts with other boxes as if they were real elements inserted just inside their associated element.

Now, if it is content that is generated, then it is not an "object" containing content, but the content itself (which happens to have some behavior similar to an element object that might contain content).

Thus, there is no "object" containing "content" (since it is content) between which the filter can place a procedural surface for content generated by a pseudo-element (i.e. "false element"). A gradient must be applied to the object, and then the procedural surface is placed between it and the content.

Answer 2

The documentation on -ms-filter -a synonym for filter- states:

An object must have layout for the filter to render.

My first guess was that the :before content doesn't have hasLayout set to true. And while it's probably not set to true, it's probably not set to false either. For starters, when I followed the hasLayout docs to force the content to get hasLayout = true (see jsfiddle) it didn't solve anything.

So I'd say it's neither true nor false. Instead, it's probably undefined. I noted in the same docs it says about the source of this property:

object.currentStyle.hasLayout

If we have a look at the W3 documentation on the content property it says:

Generated content does not alter the document tree. In particular, it is not fed back to the document language processor (e.g., for reparsing).

So, a possible conclusion would be that the generated content is not an object, as such it does not have a currentStyle property, and thus also doesn't have hasLayout set to true. This would be the reason that filters don't work on the generated content, and thus answer the question.

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At first sight I thought I had found a hint in the console of the above fiddle:

document.querySelectorAll('div')[0].currentStyle.hasLayout; 
// true

document.querySelectorAll('div:before')[0].currentStyle.hasLayout
// Unable to get value of the property 'currentStyle': 
// object is null or undefined

But as mentioned in the comments by @BoltClock: querySelectorAll cannot access pseudo-elements.

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Another hint (though -again- nothing more than a hint) that filter won't work on pseudo-elements can be found in this msdn introduction on filters, stating (emphasis mine):

Filters are applied to HTML controls through the filter property

Although I'm not sure what is meant by "HTML controls", I wouldn't expect content generated by the :before pseudo-element to be considered a "HTML Control".

Answer 3

Wow, this is a tough one.

After reviewing this chart, confirming that IE8 only likes single colons on its pseudo-elements, reading this possibly related blog article, and doing a lot of testing in jsFiddle (although, it's little in comparison to your 73?? jsFiddles), I would have to conclude that this is a bug in IE8.

IE9 can do gradients on pseudo-elements (with base64 nonsense) but IE8 is stubbornly broken.

Answer 4

Rather than using IE's filter style for this, have you considered using CSS3Pie?

This is a script for IE that adds support for standard CSS box-shadow and gradients, so you can write the same code in all browsers rather than having to have all those IE-specific styles.

Answer 5

I already gave my preferred solution (use CSS3Pie), but I'll post this as a separate answer.

The likely reason why IE8 fails to work with filter where IE7 works is because IE8 changed the syntax for filter.

filter is an IE-specific proprietary style. When Microsoft released IE8, they made a big point of trying to be "standards compliant". The "standards compliant" way of supporting a non-standard style is to give it a vendor prefix, and that is what Microsoft did.

So therefore in IE8, you need to do the following:

-ms-filter: "progid:DXImageTransform.Microsoft.gradient( startColorstr='#80ffffff', endColorstr='#00ffffff',GradientType=0 )";

IE7 doesn't support this syntax, so you need them both.

IE8 does in fact work with the old syntax in some cases. The cases where it doesn't work tend to be the ones where you use the progid: syntax. The reason for this is that the colon after progid causes it to be invalid CSS syntax, which is why MS added quotes around the whole thin for the IE8 -ms-filter version.

So the short answer is, use both versions in your stylesheets, and you'll be fine.