The basic thing I am saying is that if you call it a web browser and use the same core protocols etc., you have a practically infinite (for an individual) set of requirements to implement and will never be truly compatible.
What ideas like Tersenet and systems like Gemini propose is similar to what you say, except even simpler. HTML and ECMAScript are quite complex and have a lot of challenging expectations as far as performance and flexibility.
The idea is to try to break things off in a clean way that is feasible to implement without having to worry about a fuzzy and ever-expanding definition of what you have to support.