There is some controversy about whether IE8 will "break" the www or not. I won't get into a whole lot of detail, but in a nutshell:

When a browser requests a web page from a server, it looks at it and interprets the code based on a set of rules and standards. In most cases, the browser will take code that isn't 100% perfect and "decide" how it was meant to be and display it just fine. IE8 is supposed to be much stricter about interpreting code which may cause some sites with less than perfect code to display differently than the coder intended.

You can read about it here:

http://blogs.zdnet.com/microsoft/?p=1262