Just updated my site to newer, much more standards compliant design. My previous design was so rubbish that I had to use the IE=EmulateIE tag to force IE7 emulation.
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In the absence of an X-UA-Compatible http-equiv header, the compatibility mode is determined by the !DOCTYPE (or the absence of a !DOCTYPE, as the case may be). For a chart of which !DOCTYPE gives you which mode (in various browsers) see here:
http://hsivonen.iki.fi/doctype/ (You'll need to scroll down toward the bottom of the page.)
You can override this behavior by using a meta element to specify an X-UA-Compatible http-equiv header, like so:
(Note: IE=edge goes with the highest available version -- currently IE8 as of this posting -- or one can explicitly specify IE8.)
For more information, see here: http://msdn.microsoft.com/en-us/library/cc288325(VS.85).aspx