I'm not buying the dichotomy. Apart from anything browser-specific, JavaScript's weirdnesses ought to demolish this ideal as a real possibility. It's just a question of how broken, how crufty.

How about how open? Browser specific JS is a misnomer because blink and V8 have ~70% market share. This is a bad thing for the open web. It was a bad thing when it was IE6, and allowing chromium based browser to grow beyond 70% while Google are firmly in the driving seat is more dangerous.