Three reasons sites break in Internet Explorer 7.

This is a great write-up by Swedish blogger Roger Johansson. From my experiences over the last month, I would say he hit the hammer right on the head. This is what he had to say:

1. Sites that have an XML declaration before the DOCTYPE, making IE 6 use quirks mode, but not IE 7. Both browsers still get the same CSS, so IE 7 renders it differently.
2. Sites that depend heavily on CSS hacks that no longer work in IE 7.
3. Sites that use conditional comments to feed a bugfix stylesheet to IE without specifying a version number, thereby making IE 7 load the bugfixes and mess up rendering.