HTML4 is specified as SGML vocabulary so I don't see the problem with parsing it, especially if you leave out the script element which introduced irregularities

HTML4 is specified as SGML vocabulary so I don't see the problem with parsing it, especially if you leave out the script element which introduced irregularities. Yes SGML was seen as complex in 1996, but it's relatively sane compared to the 2020's web stack. Developing a core SGML parser (with mandatory automaton construction and tag inference) can be done in about 0.5 man-years. And developing a CSS 2 renderer should be possible in less than 49.5 years.