I think Microsoft is seriously worried about Windows Phone being not considered as a mainstream device as many websites are still not detecting (or interested in) it and thus rendering only the desktop version of the websites for IE on Windows Phone. But, if you have been noticing lately, Internet Explorer on Windows Phone 8.1 Dev Preview of GDR1/Update1 appears to have addressed this problem via a quirky trick. The user-agent (UA) string of IE on this update has changed to an interesting one; the new UA includes identifiers for Android and iOS devices too and so device-aware sites render properly as if they would on an iOS device or Android one depending the browser detection logic used by those sites. From my Lumia 1520 running WP 8.1 GDR1 DP, the UA looks like this:

Mozilla/5.0 (Mobile; Windows Phone 8.1; Android 4.0; ARM; Trident/7.0; Touch; rv:11.0; IEMobile/11.0; NOKIA; Lumia 1520) like iPhone OS 7_0_3 Mac OS X AppleWebKit/537 (KHTML, like Gecko) Mobile Safari/537

But at the same time the new UA causes some interesting problems too. An example is below, as recognized by a popular browser detection website:

For instance, if a website has mobile platform specific UI rendering logic (say, one that renders pages for native UX, visual cues, etc.), the new UA could cause confusion and make the browser detection logic become even more complex. Not only that, it could also skew the web analytics aspects of websites by accounting Windows Phone as either iOS or Android unless the analytics engines takes care of the new UA. So, if someone heavily relies on user-agent any important logic, be prepared to tweak your logic.