Horrible. We're going to end up with three separate languages: CSS, HTML, and Javascript, which will each be turing-complete programming languages with completely-overlapping featuresets, and there will be no clear reason to use one over the other.
Browsers will have to implement support for three PL runtimes. Everything will be complicated and confused.
There are tradeoffs. Further increasing the barrier to entry for new web browsers benefits the entrenched players and hurts end users by yielding fewer alternatives.
That's what people say of C++ too. Too many features makes it harder to learn a language and ramp up on codebases; they'll have different standards on what they use.
Developers can use whatever features they want, but users can only watch as their computer uses up more and more energy because it suddenly has to perform yet another build step that previously was done on the server.
Browsers will have to implement support for three PL runtimes. Everything will be complicated and confused.