Huh, so customizable select isn't part of Interop '26 and I can't even find a Mozilla bug tracking it, only their (positive) standards position.
@kleinfreund I think (and hope) @jaffathecake had this on his list and tell us more next thursday https://webdayout.com/jake-archibald/#talk
Web Day Out: Jake Archibald

Jake Archibald is speaking at this one-day event in Brighton on March 12th, 2026.

@kleinfreund here it is https://bugzilla.mozilla.org/show_bug.cgi?id=1944403 - we're actively working on it
1944403 - (custom-select) [META] Implement customizable select element

NEW (nobody) in Core - DOM: Core & HTML. Last updated 2026-03-16.

@jaffathecake Ah, cheers! Seems my bugzilla search wasn’t adequate enough to find this one
@kleinfreund yeahhhh the search is bad