This post partially is coming from an email I've received earlier, but since it is useful, so I post it here.
- IE9 = Browser Mode IE9 + Document mode IE9 Standards
- Default for the browser
- IE8 ~= Browser Mode IE8 + Document mode IE8 Standards
- Approximates IE8
- IE7 ~= Browser Mode IE7 + Document mode IE7 Standards
- Approximates IE7
- Any other combination is unsupported because it doesn’t approximate a supported browser.
- We’re specifically setting the X-UA-Compatible Meta tag to force the browser to render in IE9 unless someone has used the developer tools override. There’s nothing we can do if someone has chosen to override with the developer tools.
If you are interested in knowing more about how IE9 is determining its Document Mode (underly engine), here is the graph for it.
If you want to run multiple different version of IE from different platform ( like IE in Window Vista and XP ) all in your local machine. You can try the Windows Virtual PC VHDs.