The complexity of the markup isn't relevant. The complexity of the renderer is. And you are grossly underestimating the complexity of even an ASCII typesettin

The complexity of the markup isn't relevant. The complexity of the renderer is. And you are grossly underestimating the complexity of even an ASCII typesetting engine. RUNOFF/nroff/troff/ditroff/groff isn't even easy for a character device like a vt100. The developers of those engines reads like a who's who of computing luminaries. My point as to why rewriting a web browser is hard is that you are developing a "typesetting engine", and developing a typesetting engine is HARD.