Probably with web browsers the last 0.1% is 99.9% of the work.
Literally today I had to upload a drivers license in person yet on my phone to rent a truck. Of course Firefox failed, and even Chrome glitched, yet Chrome could be made to work with it's alternate uploader.
Web apps are >5MB piles of text, and they have a way of using pieces of browser behavior that you wouldn't expect.
But not giving up is the first step to success. I probably wouldn't use a browser in Go either, but as someone who has also been working on one of my own intermittently and knows how much opposition there is, I certainly encourage any attempts at increasing actual browser diversity.
as others mentioned, some people don't care about market share. the browsers I use day to day (Firefox, Chrome) are honestly pretty bloated and terrible, so I think others would agree that quality wise, users are starving for something better.
That said, shameless plug: if anyone wants a reasonably complete but immature ECMA262 parser in Go, I did do that. You can see how (un)finished it is here, with the wasm build: https://cleansheets.io/parser/ - source here. https://github.com/jchv/cleansheets
The truth is, I'd like to still work on this and even see if it's plausible to build a decent JIT without going too far into the weeds (I wouldn't tolerate a requirement on Cgo personally) but given that I never even pushed up an interpreter (I had an AST-based interpreter, but it was so ugly that I scrapped it :) I doubt I'll get anywhere near Opossum. Oh well.
It'd still be fun to at least get some pages rendering. Probably no chance in hell I'd ever get over to milestones I'd actually like to (like booting GMail for example.)
Did the author express that his intent was to capture browser marketshare? If not, he may not be delusional :)
This looks like an implementation from scratch which is fantastic.