I'm really excited to learn about Luakit, a browser that's extensible in Lua! https://luakit.github.io/
I've followed uzbl and Conkeror with interest but both are inactive these days.
miniblog.
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I'm really intrigued by Next, a scriptable browser. https://next.atlas.engineer/
There's definitely interest in alternative browsers designs, e.g. Conkeror and vimium.
The neat design of Next is that it's deliberately abstracting the underlying engine, giving future flexibility.