This article specifies IE 9 (Edge), IE8, IE7 and IE5 as valid HTTP-EQUIV document modes. Why was IE6 skipped? Does it render the same as IE5? My company supports IE6+, so I want to find a reliable way to test IE6 compatibility UI changes without requiring a VM.
The article specifies the compatibility modes IE (8-9) can run in. IE5(.5) equals to quirks mode. IE8 cannot run in IE6 compatibility mode. IE6 does not absolutely render the same as IE5(.5) (unless it's in quirks mode).
If you want a reliable way to test in IE6, IE7, etc., you should try something like IETester (which I found to be reliable), Spoon, etc. if you don't want to use VMs.
You should also check this SO question. As stated there:
Also, don't trust IE8 compatibility mode.