[childintime]

But does it require ES6? Javascript was quite minimal in the early days. It doesn't need a JIT, in fact I'd prefer it not to be.

[nicoburns]

The difficult bit isn't the core JavaScript support. There are a dozen engines packaged as libraries that can use for that. The difficult bit is supporting all of the hundreds of DOM APIs.

[userbinator]

Also the "undefined behaviours" used for fingerprinting and denying access to non-mainstream UAs even if they have JS support. If I remember correctly, YouTube was doing something like that.