This one will probably be more controversial than my RDFa 1.1 RFC.
It's time to drop support for XHTML 1.1 which effectively disables portions of MW's features.
https://www.mediawiki.org/wiki/Requests_for_comment/Drop_XHTML_1.1
And a commit implementing it: https://gerrit.wikimedia.org/r/#/c/63106/