I get the reasoning to forbid @import from replaceSync, but it's not clear to me why forbidding it in insertRule is desired, or necessary. We allow insertRule with @import with other stylesheets.

Looking a bit at the history this comes from #56 and such.

Do we still have hopes to eventually be able to share these stylesheets across documents? If so it makes sense to add a note to the spec saying that this is why this restriction is there.

If not, this restriction should probably be removed?

cc @bzbarsky @rakina @tabatkins