I'm rather concerned about adding svelte.config.js support to things that already have well established mechanisms for configuration.

The answer should be: you write a language that compiles to Go’s IR.

One important other option is using another programming language as your IR! If you can compile (or perhaps more accurately transpile) your language into C, then you can leverage gcc (or clang, etc) into compiling that all the way down into machine code.

However, this would lead to further divergence. Tooling that is built around the assumptions imposed by template literals wouldn't work. It would undermine the meaning of template literals. It would be necessary to define how JSX behaves within the rest of the ECMAScript grammar within the template literal anyway.

This is valid javascript! Or harmony or es6 or whatever, but importantly, it's not happening outside the js environment. This also allows us to use our standard tooling: the traceur compiler knows how to turn jsx`<div>Hello</div>`; into the equivalent browser compatible es3, and hence we can use anything the traceur compile accepts!

Doesn't require the use of transpiler or modifications to all JS tooling ever invented.

Why struggle with custom Syntax DSLs when you can use one so widely supported?

It was only pragmatic to use a tool that basically gives you that all for free.

One key advantage of 'HTML-plus' languages is that you don't actually need tooling in order to be productive — most editors give you out-of-the-box support for things like syntax highlighting (though imperfect, as JavaScript expressions are treated as strings) and auto-closing tags. Tools like Emmet work with no additional setup. HTMLx should retain that benefit.