Browser engine wars

Lunascape is a web browser that provides easy access to the three underlying rendering engines of the other major browser makers: Gecko from Firefox, Trident from IE and Webkit from Chrome and Safari. I am also aware that there are several JavaScript engines currently in use, including V8 and TraceMonkey, each unique to their host browser.

So how much of any browser is essentially different from others? Will we see rendering engines and JavaScript engines move to the status of plugins rather than core features of browsers? It's possible. And if we can have multiple rendering engines in a single browser, can we select between them as required? Say I prefer Gecko, but some pages only render correctly in Trident. I could specify to use Trident for this page and Gecko for others. Speed can be measured and used as a heuristic for selecting both rendering engines and JavaScript engines automatically, though correctness would not be an automatic selection mechanism. When different engines are present behind the scenes and automatically selected for speed, the "browser wars" are internal to every browser on the web.

Mokalus of Borg

PS - This could easily turn into a bad thing for website developers.
PPS - Rather than three or four browsers to test, there are a dozen engine combinations.