X-UA-Compatible is an IE-specific header that can be used to tell modern IE versions to use a specific IE engine to render the page. For example, you can make IE8 use IE7 mode or tell IE to use the newest available rendering engine.
X-UA-Compatible is an IE-specific header that can be used to tell modern IE versions to use a specific IE engine to render the page. For example, you can make IE8 use IE7 mode or tell IE to use the newest available rendering engine.
You can find documentation on this header on MSDN.
The most common values for this header are the following:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"><meta http-equiv="X-UA-Compatible" content="IE=IE7">
Quoting from the linked MSDN article:
IE7 mode renders content as if it were displayed in standards mode by Internet Explorer 7, whether or not the page contains a directive.
Emulate IE7 mode tells Internet Explorer to use the directive to determine how to render content. Standards mode directives are displayed in Internet Explorer 7 standards mode and quirks mode directives are displayed in IE5 mode. Unlike IE7 mode, Emulate IE7 mode respects the <!DOCTYPE>