[cssom][css-conditional] CSSGroupingRule interface is duplicated

[foolip]

The CSSGroupingRule interface is defined in two specs:
https://drafts.csswg.org/cssom/#the-cssgroupingrule-interface
https://drafts.csswg.org/css-conditional/#the-cssgroupingrule-interface

They are not identical, but the one in CSSOM looks like the one to keep.

[jugglinmike]

Just stumbled over this myself.

As @foolip mentioned, there are a few discrepancies between the two. In particular, CSSOM defines an insertRule method whose second argument is optional, while CSS Conditional Rules' version of the method requires the second argument.

This seems to be a source of confusion since Chromium requires the argument, but Firefox and WebKit do not:

<style id="s"></style>
<script>
s.sheet.insertRule('@media all {}');
s.sheet.cssRules[0].insertRule('* {}'); // throws in Chromium only
</script>

For what it's worth, an optional argument is more consistent with the insertRule method of CSSStyleSheet.

[jugglinmike]

We resolved this issue via gh-4029 though I don't have the permissions to close it.