style: Move all the fixup code into a StyleAdjuster struct.
This will allow reusing it from text styles, which we need for some corner
cases, like text-align: -moz-center and similar stuff.
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stylo: support all overflow values
overflow:clip doesn't exist, it's just called clip internally. Renamed, and added the other missing values.
I also removed the overflow newtype -- no need for extra code bloat, and it's not protecting us from much.
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Notify embedder when history changes
`WindowMethods::set_page_url` is only called when the embedder set the URL. It is not called when the page url is updated. I believe that instead we should just pass the URL to `head_parsed`.
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- [x] These changes fix#15439#15643 and #15642 (github issue number if applicable).
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Cleanup various modules and introduce generic types
Almost all the types in `values/specified` and `values/computed` share their `ToCss` implementations. The only reason they have duplicated code hanging around is a result of different specified and computed forms for types like `LengthOrPercentage`. This PR makes some of these types *generic* so that we could have a common definition and `ToCss` impl (`Parse` and `ToComputedValue` impls too, if possible).
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