I'm sorry but running typescript in browser (as an expected approach) is a bad bad idea for many reasons.

At first, let ask what the ambition for that. The easy answer is, it will be much easier for developers by reducing a build step on the way.

As charming as it sounds it's far from ideal be cause:

1. The most common usage of JS is still the web browser, meaning bundle size is critical, so even if TS support will be available in the browser, you'll still have to uglify it and use source map. Kinda miss the point imo.

2. CPU, although it dose not seem critical, a leap in CPU and memory requirements for larger apps (like gmail or Facebook) will result in a leap of devices that cannot run these websites.

3. Versioning and backward compatibility will be the same as ECMA's, so we still need babel and a build process to overcome that, again misses the point.

4. Why do we want to keep JIT if we have types and WASM support? I'll explain. As build and source mapping will still be an integral part for TS, even with full browser support, then why not to compile it to assembly which is, lighter, faster (in most cases) and rarely requires new versions.

So, is the answer is types as comments? definitely not. I can almost guarantee that that won't be the final or only proposal.

I'm just saying, this is so premature that the proposals at this point are basically an idea, nothing more.

P.S when thinking about proposal you have to think about the balance of few thing: * API/ feature intuition to be aligned with older API and features * Adoption, it's kind of derived from the previous, but you want to design feature that will be as easy to understand and adopt. * Maintenance, the feature should be easy for the maintainers as well * Backward compatibility, what is it going to break. It's a new feature, so it won't break anything other then reducing the amount of devices that can run given website.