What about OpenXPS (ECMA-388)?

I opened https://en.wikipedia.org/wiki/Open_XML_Paper_Specification and searched for "form field" and got no hits, so if nothing else the IRS couldn't use it. The Licensing section is filled with all kinds of nonsense, but I guess if it's an ECMA standard ... how bad can it be?

https://ecma-international.org/wp-content/uploads/TC46-XPS-W... and https://ecma-international.org/wp-content/uploads/TC46-XPS-W... are interesting in that they're different packaging of presumably the same data for compare-and-contrast. I will say that exploring .xps files is much easier via $(unzip) than using qpdf or friends