Applying region styling to content in a region does not work when the selector uses pseudo-elements. Using ::first-line, ::first-letter, ::before or ::after will not work. Pseudo-classes however, work just fine.
Created attachment 145066 [details] Reftest showcasing the bug
We should recheck this because pseudo-elements are now backed by Element objects.
Still repros on today's nightly (r151543)
CSS Regions were removed in Bug 174978.