As a developer, can I safely upgrade to IE8 and use its IE7 compatibility mode to test how sites look in IE7? For IE6, I have a virtual machine and it is quite inconvenient. I don't want to upgrade to IE8 and then have another virtual machine now for IE7.
Or is IE7 compatibility mode really different and I can't rely on that?
Compatibility mode in IE8 is not a 100% emulation of IE7. Security changes were not versioned, and some DOM operations were not versioned. Check out this blog post form Microsoft for a list of changes.
http://blogs.msdn.com/ie/archive/2009/03/12/site-compatibility-and-ie8.aspx
Microsoft is developing a tool called Expression Web SuperPreview to make it easier to test for the growing fauna of browsers.
Blog entries describing it by Long Zheng and The Expression Web Team.
You should try using IE Tester - a life saver. It lets you view a website on IE 5.5, 6, 7, current installed version, and 8 beta all in one program - each browser version loads in it's own tab and can be split screen etc.
The MSHTML code changed. IE7 compat mode should be pretty damn close to IE7, but... the code changed. I would still test both.
Microsoft makes Virtual PC images available for testing this.
I know for a fact that IE7 and IE8 (compatibility mode) are not the same. In my case it was CSS problem that showed up in IE7 and not in IE8
I use IE7 compatibility mode and it serves me all right. Did you try Multiple IE http://tredosoft.com/Multiple_IE instead of using a virtual machine?
Don't forget IETab for Firefox... making things that little bit easier. Though IIRC, you can only use this to test the version that's default on your system.
Seems that Microsoft has finally released Expression Web SuperPreview
It can be downloaded here
Multiple IEs was listed in another answer.
There's a successor to "Multiple IEs" called Internet Explorer Collection that is more up to date.
来源：https://stackoverflow.com/questions/712055/can-i-get-away-with-testing-sites-using-ie8-with-ie7-compatibility-on