I think it would be, for the most part, trivial to make a text-based or extremely stripped down browser on top of existing projects, if you had the contracts mapped for appropriate code generation. There are IDLs for most Web APIs, so that is a head-start.
I think this would be achievable, but not as a browser that most people would want to use.
Exactly. One of the reasons was that WebKit implements its APIs based on the WebIDL schema files (iirc Firefox does this too since Aurora).
Though the C++ code generator is a real old perl script, it's generally feasible to spoof the APIs as they would behave while providing behaviour profiles of the most commonly used web browsers (e.g. chrome/edge on windows).
The real challenge is to implement behaviour profiles that are also timing specific because some Browsers have different timings in incognito mode vs. normal mode due to how the memory is allocated directly in-RAM. That's usually how incognito mode Browsers are identified by recaptcha.