First thing I see when I try to visit Startpage, for probably the first time ever, is "Our system has detected the type of high-volume traffic coming from your internet connection that is normally associated with bots and scrapers." WTF. Total BULLSHIT. I have accessed that site precisely ZERO times from this IP. Now it will be one, and the last time I ever try.
Google at this point kind of controls the www. Now, strictly speaking that statement is not true, but it now feels as if Google sits in so many areas that are important for the www; chrome is just the most obvious one.
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.
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.