Support shorthand property "all"

[upsuper]

all is a shorthand property defined in css-cascade spec to set almost all of properties at the same time to a given CSS-wide keyword. So it is mainly a style system thing, which doesn't require any layout support.

[SimonSapin]

The naive way to implement this would be literally as described: expand it at parse time to hundreds of longhand declarations and cascade all of these.

But that sounds expensive, and it’s probably possible to be smarter about it. What does Gecko do?

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Note:

https://drafts.csswg.org/css-variables/#defining-variables

Custom properties are not reset by the all property.

[emilio]

If we see the all property it seems we could do the following:

• If the value is inherit, drop all our computed styles right now, and just clone the parent one, which is easy (except we need to unicode-bidi and direction if they were specified).
• If the value is initial, do the same, just cloning from the default computed values instead.
• If the value is unset, clone individual structs as needed from either the default computed values or the inherited values depending on whether it's a reset or an inherited struct.

[SimonSapin]

If we see

There’s two separate phases: parsing, which is when other shorthands are expanded into their longhands, and cascading. So far, longhand declarations expanded from the same shorthand declaration casacde separately. But maybe we can treat all differently and have an All variant in the PropertyDeclaration enum? (Which so far only contains longhands and custom properties.)

[emilio]

Yeah, that looks reasonable to me, and it's what I had assumed when writing my comment above.