[css-pseudo] Should the "first formatted line" propagate into a different BFC? #11038

Split the issue from #10990 (comment) to discuss easier.

Blink I think currently has issues with ::first-line propagating into elements which establish independent formatting contexts, when arguably it shouldn't, when I last read the spec here somewhat questionable.
E.g.
https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=13169

@bfgeek @fantasai