It's not a terrible idea but it's difficult to make it work. First, they have to agree to work together. Then they have to decide which language to implement

It's not a terrible idea but it's difficult to make it work. First, they have to agree to work together. Then they have to decide which language to implement and write a spec. It must also support what browsers do in JS (DOM manipulation, APIs, etc.) Finally they have to implement it in an interoperable way. All of that costs money and what is the advantage over JS to justify those costs?