Doing some network traces using a transparently in-line Windows Phone, I got my first glimpse of the User-Agent string Internet Explorer advertises, it’s somewhat ambiguous to say the least:
Mozilla/5.0 (Mobile; Windows Phone 8.1; Android 4.0; ARM; Trident/7.0; Touch; rv:11.0; IEMobile/11.0; NOKIA; Lumia 925) like iPhone OS 7_0_3 Mac OS X AppleWebKit/537 (KHTML, like Gecko) Mobile Safari/537
It literally lists every major browser vendor known. I can only imagine Microsoft do this to guarantee compatibility, ensuring that the site always returns some content, but this will certainly break any layer seven devices that happen to be making decisions on User-Agents, as Windows Phone is going to match them all.
Still, I’ve not encountered any broken pages just yet…