There's a new browser in town: Nyxt.
It is free software.
It is intended to be modified by the user, perhaps even rewritten.
From https://nyxt.atlas.engineer/ [nyxt.atlas.engineer]:
Built-in programmability.
Use the built-in REPL to program Nyxt. Run short scripts, and try out new workflows. Everything in Nyxt is fully extensible and modifiable.
It is written in Common Lisp.
Is this the browser we programmers have been waiting for? The one we can modify to out wildest dreams?