This is one painful issue that took me too long to figure out, what a waste of time! I built a simple html5 webpage with a video & audio, works perfectly in VS and in localhost mode, but once I deploy in intranet, does not work anyme in IE9! Same site works well in other browsers like Chrome. Couldn't find anything in web searches.

Finally F12 tools helped in IE. The console pointed out this error: "Display intranet sites in Compatibility View" is checked. So the compatiblity mode turned on automatically and hence none of my HTML5 specific code was recognized. I tried to find where it is checked in the internet options dialog, but could not find out.

The solution is to force use of IE9 in intranet environment by using below tag:

<meta http-equiv="X-UA-Compatible" content="IE=9" />