<!doctype html>
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
This makes each version of IE use its standard mode, so IE 9 will use IE 9 standards mode. (If instead you wanted newer versions of IE to also specifically use IE 9 standards mode, you would replace Edge by 9. But it is difficult to see why you would want that.)
For explanations, see http://hsivonen.iki.fi/doctype/#ie8 (it looks rather messy, but that’s because IE is messy in its behaviors).