Why just the US?
If the OpenTaxForms approach is useful for the tax forms of any other nation, I'd love to hear about it.
Why process instructions and publications rather than just the forms?
Instructions and publications are full of useful information to assist taxpayers in filling out tax forms. For example, forms often require computations from worksheets embedded in instructions.
Will the IRS be upset?
Converting forms, instructions, and publications to software is essentially what TurboTax, TaxACT, and other vendors of tax software do.
Will Adobe be upset?
This is just what Adobe suggests that we do for forms in the PDF file format, according to PlanetPDF : Adobe "suggests that users should ... take advantage of the XFA structure to produce forms as HTML5 rather than PDF."
Why do you use only pure-Python libraries, which are slower than native executables?
For simple and reliable installation of the software, to remove barriers for new users.