IE9 has a way to put it into IE8 compatibility mode. Do these kinds of sites not work in Compatibility Mode? Press F12 to switch the mode. It's a good option for using a better version of IE while still supporting older sites.
Not only don't they capture all the quirks, but they introduce new ones. I had an issue a year back, I can't remember the exact details unfortunately, where IE8 in compatibility mode for IE7 was causing some weird behaviour that didn't happen in either IE7 or 8.
To make things even more confusing there is both a "browser mode" and a "documents mode", both containing all the versions of IE7,8,9 plus documents mode containing IE9-compatibility-mode whatever that is. Setting any combination of the two options is valid.
There's a sad story waiting to be written about the developers that have had to spend so much time deliberately introducing bugs into IE10 to make this feature work.
Nah, they just plug in the old Trident renderers. "Browser mode" changes the user agent, and "Document mode" changes the renderer. The JS engine remains the same, which is why you always need to test in the real deal.
