CSSOM: Whole ton of things
CSSOM is now starting to be useful!
Based on #14190. Only the <s>last commit</s> last two commits need review.
cc @xidorn . This doesn't change the style API, but adds useful methods.
part of #11420
This adds:
- `insertRule()` and `deleteRule()` on `CSSStyleSheet`, `CSSGroupingRule`
- `.style` getters on link and style elements
- Keyframes-backed `CSSRules` and `CSSKeyframesRule.cssRules`
- `CSSGroupingRule.cssRules`
- `prefix` and `namespaceURI` attributes of `CSSNamespaceRule`
- Fixups regarding parent stylesheets
r? @SimonSapin
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