How do we find which are the pending style sheets ?

There are situation where in JS code we need to know the height/width
CSS value set using linked/imported style sheet. If it takes some time
to load the style sheets, using JS code how do we find which are the
style sheets that have not completed loading/parsing?

PS: I think this is the reason for regression
https://bugzilla.mozilla.org/show_bug.cgi?id=404551#c6