Fyrd/caniuse — pull request #437 by YetiOr (2014-02-05 03:20:36+00:00)
Title: IE 9 Mobile on WP 7 doesn't support position:fixed

http://msdn.microsoft.com/en-us/library/windowsphone/develop/ff462082(v=vs.105).aspx
Unsupported features in Internet Explorer for Windows Phone OS 7.1 :
• Positioning elements in a fixed position relative to the layout viewport.