Microsoft's stop-gap solution to fix IE 10 & 11's viewport on Windows Phone 8. I've also added another condition so it won't run on other browsers that spoof the user agent. Details: http://mattstow.com/responsive-design-in-ie10-on-windows-phone-8.html

wp8-ie10-fix.js

(function() {
	if ("-ms-user-select" in document.documentElement.style && navigator.userAgent.match(/IEMobile/)) {
		var msViewportStyle = document.createElement("style");
		msViewportStyle.appendChild(
			document.createTextNode("@-ms-viewport{width:auto!important}")
		);
		document.getElementsByTagName("head")[0].appendChild(msViewportStyle);
	}
})();

It worths mentioning that this code will not work on "Edge browser - Windows 10 Mobile" since the user-agent is now different:

Mozilla/5.0 (Windows Phone 10.0; Android 4.2.1; Nokia; Lumia 520) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2486.0 Mobile Safari/537.36 Edge/13.10570

Needs a little bit of tweaking to make it work.