It's not entirely trivial to write out correct HTML. In addition to properly closing tags, you need to deal with optional end tags (many get this wrong), optional start tags, the odd comment syntax, weird exceptionality wrt escaping esp. in script tags, context-dependant validity such as no block-level elements in inline elements (or nested a tags, or tags whose "type" depends on their attributes).

It's obviously easier to write than to read; but it's definitely easy to make a mistake. There's a lot of illogical cruft that's accumulated in HTML; so even a careful implementer might make a mistake (and might not detect it since most other implementations are so liberal).

I'm was talking about the 90s.