These are basically $htmlpairsStatic and the tags that are in $htmlsingle but not $htmlsingleonly ( $htmlpairsStatic + $htmlsingle - $htmlsingleonly).
This will allow using all of the paired HTML tags that can be used in normal wikitext (and thus can be used in parsed messages).
There is an in-progress patch for this at https://gerrit.wikimedia.org/r/#/c/85153/ . However, it needs to be amended, and the suggestion is to export this tag list to JavaScript.
Version: 1.24rc
Severity: normal