In quirks mode, it acts like IE 5.
Implements most of CSS 2.1 goodness.
To turn off compatibility mode and revert it back to IE 7 behavior, put the following tag in the head:
<meta http-equiv=”X-UA-Compatible” content=”IE=edge” />
Now, if you want a really good HTML5 ready engine to run on your old IE browsers, put the following tag:
<meta http-equiv="X-UA-Compatible" content="chrome=1" />
That will render pages as in chrome, even as you hit them with IE.