So consider this extreme case: suppose we take a large chunk of plaintext and compress it with something like DEFLATE (but in a tokenizer friendly way), so that it uses 500 tokens instead of 2000 tokens. For the sake of argument, say we’ve done our best to train an LLM on these compressed samples.
Is that going to work well? After all, we’ve got the same information in a quarter as many tokens. I think the answer is pretty obviously “no”. Not only are we using a small fraction as much time and space to process the information, but the LLM will be forced to waste a lot of that computation on decompressing the data.
For example, adding a repeated word somewhere in a completely different part of the document could change the dictionary and the entirety of the compressed text. That's not the case with the "compression" offered by converting HTML to Markdown. This compression more or less preserves locality and potentially removes information that is semantically meaningless (e.g. nested `div`s used for styling). Of course, this is really just conjecture on my part, but I think HTML > Markdown is likely to work well. It would certainly be interesting to have a good benchmark for this.
That's not to say that I think that converting to markdown is pointless or particularly harmful. Reducing tokens is useful for other reasons; it reduces cost, makes generation faster, and gives you more room in the context window to cram information into. And markdown is a nice choice because it's more comprehensible to humans, which is a win for debuggability.
I just don't think you can justifiably claim, without specific research to back it up, that markdown is more comprehensible to LLMs than HTML.