[cssom] CSSImportRule.styleSheet when not loaded yet

[SimonSapin]

https://drafts.csswg.org/cssom/#the-cssimportrule-interface

The styleSheet attribute must return the associated CSS style sheet.

Note: If loading of the style sheet fails its CSS rules list is simply empty, i.e., an @import at-rule always has an associated CSS style sheet.

What happens when the imported stylesheet has not finished loading yet? Should the getter block? Or does the does the associated stylesheet have an empty rule list, like for a failed load?

CC @upsuper @Manishearth @heycam