[tmp10423288442]

HTML is famously forgiving as well - that's the whole reason XHTML failed, because one typo in the latter will make your entire web page fail to render with an error. Markdown is probably a little more forgiving, which mattered more with previous-gen LLMs with small context windows. Any near-frontier model should have no problem generating valid HTML.

Also a lot of LLMs are trained specifically to expect Markdown in their instructions, OpenAI's models in particular (Anthropic expects more pseudo-HTML/XML, but that is different from real HTML/XML).