Your "Accept: application/x-Gemini-markdown" might as well be "Accept: text/html-without-ads-or-scripts-please" and then there is no need to write an additional document parser.
Well the html format does allow for JavaScript, images, etc. Not to mention it allows for arbitrary document hierarchy. it would be a large cognitive burden on users who are publishing content to remember what is the gemini-supported subset of html that is allowed. It would also be hard to standardize the subset and prevent the subset from growing.
What Gemini got right is that simplifying and limiting the publishing format prevents bloat.
What Gemini got right is that simplifying and limiting the publishing format prevents bloat.