Interesting. I didn't know that someone had started working on form support (in 2016 or 2012). PDF.js started in Mozilla Research as a demonstration, and benchmark, for JS and Canvas capabilities. I don't know how much work adding full form support would be. PDF is a complicated format and I imagine there are a lot of corner cases.
(I work for Mozilla but am not involved with PDF.js or PDFium.)
Acroform support (so not the proprietary Adobe Livecycle XFA stuff) is relatively straightforward once you understand the underlying PDF specs. It's been around since PDF 1.2 (1996!!)
At the rate I see development moving, pdf.js should be able to handle, say, every IRS form well before the end of the year, perhaps much sooner.
If I had the skills or resources to help now, I would.
I still don't understand why Mozilla has apparently not reached out to the larger business/governmental universe to see if they could partner in moving pdf.js forward. A single large insurance company might spend millions each year on licensing and implementing pdf "solutions". I can only imagine what the US government uses (or could use.)
Has anyone from Mozilla spoken to 18F?
Perhaps even a open (and not proprietary) JSON (instead of XML) based form standard for "next-generation" forms could be implemented, with PDF.js at its core. (see https://github.com/modesty/pdf2json I linked to before for an element of that...the author is an employee of Intuit...perhaps it's already being used by them for their online tax software?).
(I work for Mozilla but am not involved with PDF.js or PDFium.)