[css-conditional][cssom] Why CSSConditionRule?

[svgeesus]

from https://lists.w3.org/Archives/Public/www-style/2013Jun/0331.html by @zcorpan

In http://lists.w3.org/Archives/Public/www-style/2012Aug/0749.html it
was decided to mint the CSSConditionRule interface and have
CSSSupportsRule and CSSMediaRule inherit from it.

I don't understand the usefulness of exposing a new property on
CSSMediaRule that does the same thing as mediaText. Also, conditionText
has different requirements for CSSMediaRule and CSSSupportsRule. Why do
we want to have the same attribute for things that are different?
https://bugzilla.mozilla.org/show_bug.cgi?id=814907#c3 suggests
@-moz-document is again different.

I think it would be better continue with just having .mediaText on
CSSMediaRule, drop the CSSConditionRule interface, and let
CSSSupportsRule and CSSDocumentRule have .conditionText with the
interface-specific requirements.

[zcorpan]

I believe this has been implemented, so I would be ok with closing (but it would be nice to make sure there are tests for this).