[cssom] CSSImportRule.styleSheet when not loaded yet #453
Description
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
@importat-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?