meta http-equiv="X-UA-Compatible" Element
Amongst ie8‘s stellar accomplishments (nice logo) comes the proprietary header x-ua-compatible which grants magical control over the rendering mode the browser uses.
Don’t use it! For reals! But if you find yourself in that fun situation that you have no control over, change the header on your server so you do not get an html5 validtion eror.
<meta http-equiv="X-UA-Compatible" content="IE=edge" />