Or, put it another way, a html renderer that can open most web pages, but has a different programming/scripting language that it can interpret during runtime or on page loads, instead of the javascript engine.
I suppose that java applets, flash and activeX were attempts in these directions, but they were things you had to install on top of the browsers, so not quite the same thing? I’m imagining something like web pages using Lua, since it’s lightweight, to make them dynamic.
I would just replace it with a compile target - aka WASM. Then the web is not tied to one language but many can be built on top of it. WASM does need to more features then what is currently available to fully replace JS. DOM access for one, and probably a GC to reduce the amount of runtime you would need to ship (at least for languages that use a GC).