@tek @zompus There already was FreeFileFillableForms, the govt-supplied but TaxPrepCo-designed system. I had to use it a few times for my partner's returns, and it was foul.
Because it had clearly been developed with a "the government must not provide a value-added service!" mindset, its idea of helpful error messages were XPath validation warnings for missing fields. I am a big ol' XML nerd tho, so we got the filings done despite that