> From the browser's point of view, it would essentially treat the type annotations and type declarations as comments.

V8 still has to updater its parser. Other tools that handle JavaScript parsing need to do that as well.

> You'd be able able to use typescript autocomplete and linting in your editor, and just serve the files to your browser with a static http server.

Nope, it's not using TypeScript directly, but just another language very similar to TypeScript.