http://msdn.microsoft.com/en-us/library/jj676915(v=vs.85).aspx

Understanding legacy document modes

Use the following value to display the webpage in edge mode,which is
the highest standards mode supported by Internet Explorer,from
Internet Explorer 6 through IE11.

<Meta http-equiv="x-ua-compatible" content="IE=edge">

Note that this is functionally equivalent to using
the HTML5 doctype. It places Internet Explorer into the highest
supported document mode. Edge most is most useful for regularly
maintained websites that are routinely tested for interoperability
between multiple browsers,including Internet Explorer.

Note Starting with IE11,edge mode is considered the preferred document mode. (In
earlier versions,it was considered experimental.) To learn more,see
Document modes are deprecated. Starting with Windows Internet Explorer
8,some web developers used the edge mode Meta element to hide the
Compatibility View button on the address bar. As of IE11,this is no
longer necessary as the button has been removed from the address bar.
Because it forces all pages to be opened in standards mode,regardless
of the version of Internet Explorer,you might be tempted to use edge
mode for all pages viewed with Internet Explorer. Don’t do this,as
the X-UA-Compatible header is only supported starting with Internet
Explorer 8.

Tip If you want all supported versions of Internet
Explorer to open your pages in standards mode,use the HTML5 document
type declaration,as shown in the earlier example.

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> What does <meta http-equiv=”X-UA-Compatible” content=”IE=edge”> do?