Achieve parity with CSSOM #6
Description
The CSSOM package includes basic support for several interfaces, but only MediaList seems to offer methods we still need to implement for parity. We should also add initialization of CSSImportRule, CSSKeyframeRule, CSSKeyframesRule through the parser.
It is worth noting that CSSOM also supports CSSDocumentRule, CSSHostRule, CSSValue and CSSValueExpression interfaces. These are either experimental or deprecated and see little use, so I believe we can leave them unimplemented.