[pcwalton]

"If Mozilla were to switch to WebKit, rewrite its DOM implementation to be 20x faster, and then release that -- that would be absolutely stupendous!"

Rewriting WebKit's DOM implementation to be 20x faster wouldn't be possible without rewriting WebKit. The DOM implementation is one of the most central parts of any rendering engine. We're working on doing that (Servo), but not by building on top of WebKit for precisely this reason.

"Much in the same way that the Chrome team created a new JavaScript engine that was much faster than Safari's JS engine."

They did that by replacing JavaScriptCore wholesale, rather than building on top of it. This was only possible because JavaScript is a standard, not defined by JavaScriptCore's implementation. If JSC had had a monopoly and the Web started relying on JSC's quirks, then V8 might never have happened.