[c-smile]

> Is there a good reason browsers could and or should not support ts out of the box?

Yes, there is a reason. Most critical requirement for the "language-behind-web-UI" is to be efficiently parseable (parsing speed and memory). So it should have one-pass compiler/interpreter without the need of building AST (syntax tree). Speed of execution and type strictness is on the third and tenth places correspondingly.

JS matches that basic requirement but TS is not.

[YmiYugy]

There are proposals with some traction that would enable webbrowsers to parse TS, but simply ignore the types. I think it's a bad idea, because tooling handles that perfectly well and I think browsers should generally avoid building functionality that could be equally well provided outside it, but I guess enough people have been asking for it, so it might happen.