Major LLMs (e.g. ChatGPT) have a copy button for their output, yielding markdown that is already being consistently rendered by other apps, reflecting what was consistently rendered as HTML. Presumably LLM vendors have built a deterministic way of generating spec-compliant HTML and their well-defined dialect of Markdown, otherwise their chat UI output would not render consistently.

I have certainly seen LLMs generate broken Markdown, so it’s presumably a “hope it works” thing.