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:
<meta http-equiv="X-UA-Compatible" content="IE=edge" >
(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