Back when HTML was invented, it was based on SGML. In retrospect, this was a fundamental design flaw. SGML has all sorts of fillips that were intended to make documents easy to type (back in the pre-GUI era) but which in practice make them hard to parse. SGML parsers, even parsers built into expensive commercial software, were famous for not reliably interoperating. The HTML5 spec abandons SGML entirely because, as it turned out, even those fillips of SGML did not make it possible for browsers to consistently agree on how to parse HTML in the wild.

It would have been better for everyone if HTML had used a bog-simple Lisp-like format from day one, but without a time machine we’re stuck with these kludges of history.