I haven't given this a lot of thought, but hear me out for a moment:
When people say "the complexity of modern web technologies make it almost impossible to create a new web browser from scratch", what they really mean is "a new web browser that supports everything Chrome does".
I wonder whether that's even necessary. After all, the web _should_ be built with progressive enhancement in mind.
But who am I kidding, right? Most people only test their sites in Chrome these days.