Wow I like it, simple like Flask, but with the HTML in the same file (as opposed to the jinja templates) which is a pattern I really like!
Makes me wonder if you could integrate this ambition to do the web-stuff in your Python file with the FastAPI/Pydantic BaseModel situation. Like a HTML templating library made out of Python data classes (or Pydantic BaseModels).
Either way, I respect the ambition! :) Always cool to see web stuff with Python.