[css-cascade-6] Add implicit scopes

[andruud]

Resolves #6606 #7349

[andruud]

@mirisuzanne Please have a look.

I felt that I had to reorganize a bit here to make a natural place for "implicit" scopes to live.

Description of changes:

• A scope should probably have a single scoping root. However:
• A single @scope rule can produce multiple (explicit) scopes. Or a single implicit scope.
• Removed the circular or near-circular sentence "selectors are scoped to the given scope, with the :scope element being the scoping root" which Emilio complained about at some point (somewhere).
• Avoid selector-4's definition of "scoped" for now, because it does not clarify what it means by "descendant", and does not account for scoping limits.
• Beautify the grammar part of the spec.

Hopefully it's more or less to your liking.

[mirisuzanne]

@andruud I think it's a big improvement, thanks. I suggested a change to the syntax production, allowing lower boundaries when the root is implicit. Otherwise, it looks good to me.

[mirisuzanne]

After moving this above in the previous PR, I moved it back down! 😅

[andruud]

:-)

[andruud]

@andruud I think it's a big improvement, thanks. I suggested a change to the syntax production, allowing lower boundaries when the root is implicit.

Ah, right, that didn't occur to me. But then we need another fixup: PR: #8466