https://www.w3.org/Bugs/Public/show_bug.cgi?id=17011, given that's apparently appropriate nowadays.

CSS should somewhere (hence the lack of any tagged spec!) define which subresources block the load event; at least when that bug was filed IE, Firefox, and Opera blocked the load event on background images and on web fonts loading (which is therefore related to #1082), Safari and Chrome (and this is pre-fork!) didn't, blocking only on @import.

I believe someone claimed recently that Edge matches Safari and Chrome, making Firefox the odd one out. Someone should, however, test that.