If you support IE, for versions of Internet Explorer 8 and above, this:
<meta http-equiv="X-UA-Compatible" content="IE=9; IE=8; IE=7" />
Forces the browser to render as that particular version's standards. It is not supported for IE7 and below.
If you separate with semi-colon, it sets compatibility levels for different versions. For example:
<meta http-equiv="X-UA-Compatible" content="IE=7; IE=9" />
Renders IE7 and IE8 as IE7, but IE9 as IE9. It allows for different levels of backwards compatibility. In real life, though, you should only chose one of the options:
<meta http-equiv="X-UA-Compatible" content="IE=8" />
This allows for much easier testing and maintenance. Although generally the more useful version of this is using Emulate:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />
For this:
<meta http-equiv="X-UA-Compatible" content="IE=Edge" />
It forces the browser the render at whatever the most recent version's standards are.
For more information, there is plenty to read about on MSDN,
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