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
Source-Repo: https://github.com/servo/servo
Source-Revision: 996756687cd741ad0d30029022638bdcceecb563
