Now you perhaps realize why I haven't pushed this further.
Much of the problems is inside tkhtml which is not rendering correctly, but there can also be "special" elements like gradients that only a certain vendor supports. I've seen Microsoft have some acces to gradient drawings. So beware for gradients. You need to look at the html to figure out.

Many things are not implemented by me, like url links, /me etc. There could also be some differences of the html templates. You need to look inside to figure out.

If you can isolate specific rendering problems to the tkhtml widget providing simple examples, then add these to their bug tracker. Problems are usually fixed very fast.

Then it doesn't hurt to look at the code in ChatTheme.tcl. Only very little pain perhaps ;-) There you see how the templates are being used.