When I said "agreed upon format" I was thinking of something like JSON to talking to the server. I didn't think of various tabs on a browser having to interact.

You probably know more about how the browser works than I do, but I think I see what you're saying.

Suppose we implemented a browser that can allows front ends coded in RUST. In my mind, the browser just gives the RUST app its own tab, with its own memory and so forth.

I am not proposing mixing up languages for the a given front end, or for components within an application or anything. Not because I think that would be wrong, but because I haven't thought it through.