Immutable CSSOM
This PR is intended to add basic support for all CSSOM interfaces, with the ability to index `document.styleSheets` and css rule lists, and serializing individual css rules. Handling individual interface methods for CSSRule subclasses can probably be done with easy/medium bugs.
Mutation safety isn't dealt with here; if the css rule list is mutated the CSSOM will be in an inconsistent state. I intend to deal with this via zero sized tokens, see https://groups.google.com/forum/#!topic/mozilla.dev.servo/AnxJoVmtMXQ . I'll handle that when I start making the CSSOM mutable. (Getting the immutable bit landed first opens this up for easy bugs)
This doesn't really change style aside from adding an extra arc in the CSS rule list as discussed in the linked thread. So far this same design can be used by stylo as well when the time comes.
f? @SimonSapin @emilio
cc @upsuper
part of #11420
Todo:
- [x] Stubs for rest of the CSSRule subclasses
- [x] <s>ToCSS impls for CSSRules.</s> May make into easy bugs and stub out in this PR https://github.com/servo/servo/issues/14195
- [x] Cache CSSStyleSheet on the relevant node
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Added parsing/serialization for text-emphasis-position
Implemented parsing/serialization for text-emphasis-position
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- [ ] These changes fix#13850
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Implement border-image shorthand
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Implementation of border-image shorthand.
r? Manishearth
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Update CSSRule naming from 'CSSRule' to 'CssRule'
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Update CSSRule naming from 'CSSRule' to 'CssRule'
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Adds scroll-snap-type shorthand property
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Follow up to #14017
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r=Manishearth
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And make empty list the default value of MediaList.
This commit also removes Option wrapper of Stylesheet.media because
a stylesheet should always have an associated media query list.
Rename media_queries::MediaQueryList to MediaList
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`MediaList` is the name of the interface for "collection of media queries" used in [CSSOM spec](https://drafts.csswg.org/cssom/#the-medialist-interface), `MediaQueryList` happens to mean something different in [CSSOM View spec](https://drafts.csswg.org/cssom-view/#the-mediaquerylist-interface), which also leads to naming conflict in dom code. So I think it's better renaming it to `MediaList`.
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