On Mar 20, 2009, at 10:47 AM, Matt Graham wrote:

> Also, if it truly uses a different engine for rendering, it'll break
> all the sites that are IE-only, and a bunch of people will have a
> large number of cows.  So yeah, that's probably not going to happen.
> Unless they put in a "switch between old rendering engine and new
> one" button, which will merely keep the old problems around.

That's already the plan for IE8.

"Microsoft's solution is a compatibility button that users can press  
to make IE8 render the page similarly to how IE7 would."
http://arstechnica.com/microsoft/news/2009/03/mix09-internet-explorer-8-released-progress-unmistakable.ars 
  (in the 'Standards' section)