[selectors] webkit-prefixed pseudo-elements are always treated valid in WebKit and Blink #2156
Description
There is a webcompat issue raised against Firefox that Firefox treats rules like the following invalid which breaks some websites:
.promagnifier, .prosettings, .searchsettings,
.search input[type=search]::-webkit-search-decoration,
.search input[type=search]::-webkit-search-cancel-button,
.search input[type=search]::-webkit-search-results-button,
.search input[type=search]::-webkit-search-results-decoration {
display: none !important; }This is a reasonable behavior since Firefox doesn't support those webkit-prefixed pseudo-elements, and thus, according to the spec, it should treat the whole selector list invalid.
However, at least Blink only supports -webkit-search-cancel-button, and doesn't support any other pseudo-element here either, but it doesn't treat the rule invalid.
Then I did some test and I realized that in Blink and WebKit, all pseudo-element prefixed with -webkit- is treated valid, as can be shown with the following testcase:
<!DOCTYPE html>
<style>
div {
width: 100px; height: 100px;
background: red;
}
div::-webkit-something-invalid-12345, div {
background: green;
}
</style>
<div></div>
<script>
alert(document.styleSheets[0].cssRules[1].cssText);
</script>I'm not sure what should we do, then.
Should we spec this behavior that any webkit-prefixed pseudo-element should be accepted even if it's unknown? Or should we push WebKit and Blink to change their behavior here?