Internet Explorer 8 introduced a button beside the address bar that looks like a rectangle with a break through its middle. If you click it, Internet Explorer reports, "Compatibility View on" but the button does not clearly indicate if compatibility view is on or off. Compatibility view is Internet Explorer 8 pretending to be Internet Explorer 7. To prevent your website visitors from clicking this button, simply make it go away:
Include either the following meta element (which in invalid in HTML5) on your page <meta http-equiv="X-UA-Compatible" content="IE=Edge"> (before any script elements!) or set the following HTTP header on your page: X-UA-Compatible: IE=Edge
[Source, hsivonen.iki.fi, Activating Browser Modes with Doctype]
