There's some weirdness around android browsers refusing to load html from the phone itself on security grounds. The OP uses a "progressive web app" which seems to be the proper way to do this at some point in the past, but firefox has killed that for some reason. Thus the most difficult part of the project seems to be persuading my phone to render html without copying the files to some server (or running a fileserver on the phone itself, to look at from the phone, which seems absurd).

What I can offer for people stumbling down the same path:

- Obsidian works fine as for ad hoc authoring of markdown while walking around

- It is known on stack overflow that markdown can't be sanely parsed into an AST

- That knowledge is kind of a branding misfire, the 'markdown' one is looking for is here https://github.com/commonmark/cmark

- That cmark binary + some foreach-file style script will turn markdown into html or xml. If you choose makefile and put spaces in the name of files you'll have a moderately bad few minutes cursing your tools

- Given that output xml/html, you can build whatever other html you see fit. I'm doing that with xslt but people are telling me not to

- That html can be written back to the obsidian vault and opened by a phone browser (at least if it's a single file)

- Obsidian doesn't really do headless, but leaving it running on one machine which repeatedly calls "make" to write the *.html files back into the vault works fine

- CSS thus far appears to be required, I was really hoping to annotate the html instead

First 90% done here, second 90% to go.