Mentioned by Vito on irc while complaining about changes to Special:Undelete (T182398)
On MonoBook (Apex theme), a read-only TextInputWidget has the text marked grey to mark the field read-only. However this is not true on other skins (Tested Vector and Timeless).
This is really odd to me. It may make sense to make such widgets grey to mark them read-only (Doesn't really on Special:Undelete, but I could imagine it would in other cases). However it should be consistent across skins.