https://html.spec.whatwg.org/multipage/semantics.html#link-type-stylesheet

Once a resource has been obtained

See https://bugzilla.mozilla.org/show_bug.cgi?id=958415#c6 onwards

Also https://www.w3.org/Bugs/Public/show_bug.cgi?id=29464 for the bug on the CSSOM side (I suppose both HTML and CSSOM need to be clearer here).