[css-scoping] Consider making ::slotted and :host take a single argument.

[emilio]

As far as I know, no engine implements ::slotted with multiple arguments, and keeping it with a single argument allows to optimize it more easily (see https://bugzilla.mozilla.org/show_bug.cgi?id=1425757).

[emilio]

This also prevents issues about defining specificity of the selector. See #1027 and #1915.

[emilio]

This request also applies to :host.

[css-meeting-bot]

The Working Group just discussed Consider making ::slotted and :host take a single argument., and agreed to the following resolutions:

• RESOLVED: Accept proposal in https://github.com/w3c/csswg-drafts/issues/2158

The full IRC log of that discussion

<dael> Topic: Consider making ::slotted and :host take a single argument.
<dael> github: https://github.com//issues/2158
<dael> emilio: Single argument prevents discussion about specificity and it's simpler to organize.
<dael> ???: Is this the same as matches?
<dael> emilio: Right.
<nainar> s/???/rune_
<dael> rune: I don't know how much sense it makes to have this implicit matches thing
<dael> TabAtkins: Spec was clear. People have problems reading work like default.
<dael> emilio: Spec was clear but impl are shipping with something else.
<dael> TabAtkins: I don't care if we do 0 spec or spec to an arguement. You can always nest a matches in there.
<dael> Rossen: I'm hearing people in favor. Objections?
<dael> RESOLVED: Accept proposal in https://github.com//issues/2158