HTML browsers have been caught in a race to the bottom ever since Mosaic became popular. In fact, that popularity was the result of Mosaic's tag-soup parsing and simple-minded semantics. It was a triumph of Low Tech.
So, you're not likely to find browsers written for the "Mosaic market" getting any signficant portion of HTML syntax right.
The alternative is an SGML browser, such as
(URLs when I get a Round Tuit. Sorry about that.)