[css-contain-3] Style feature parsing should disallow !important

[anttijk]

The text says

The syntax of a <style-feature> is either the same as for a valid declaration [CSS-SYNTAX-3], a supported CSS property, or a <custom-property-name>.

and a valid declaration allows !important so I suppose it is allowed. There are even WPT subtests for it (in https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/container-queries/at-container-parsing.html and https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/container-queries/custom-property-style-queries.html).

Using !import here is useless and confusing though so should not be allowed.

It would probably be better to define the syntax in terms of a property name and an optional <declaration-value> instead, analogous to normal container queries. This will also easily expand to allowing range syntax for registered custom properties.

[anttijk]

@lilles @andruud

[Loirooriol]

Duplicate of #7413