[css-values-5][cssom] How should <url> modifiers be used when fetching a stylesheet from an @import rule?

[weinig]

As far as I can find, stylesheet fetching uses the algorithm in cssom, fetch a style sheet and does not include use of any of CSS Values 4, fetch a style resource.

As such, for an @import rule, which uses the <url> production so can have URL modifiers, its not clear how exactly those modifiers should apply to the fetch.

(cc @noamr, @annevk, @tabatkins, @fantasai)