The most broken inconsistency across web browsers is printing, print preview, print-related CSS, etc. No print media related features made it onto this list :(
Anyone could submit a proposal. Look out for that part of the process next year.
I vote for them basically every year and have submitted in the past. Every year I'm told that there's no way for WPT to properly test print stuff so those proposals are pointless
You can suggest an "investigation area", which is essentially "how can we make the tests better".