The result might be interesting, is this something you want to/can share?
Unfortunately, no.
Definitely backwards compatibility is a core feature that we can’t afford to lose, but what about a codemod script that takes a wiki and rewrites it to be compatible with modern CSS?
I guess TWX might eventually come to save the day
May I add another suggestion, Thomas Elmiger’s Plugins
A number of items on this page show layout options Tix being the latest, simpler versions such as Btx Bottom toolbar experience, Lox and Tix - A new kind of tiddler experience – incredibly flexible and configurable thanks to several parts: top, header, the standard tiddler body and a footer.
See Tix configuration for example.