Recently I encountered an issue on a project where html5 videos that had been working fine were having playback problems on Windows 7 systems in Internet Explorer 10. Further testing showed that IE11 had the same issue as well. Some time on the search engines found others with issues but no solutions.
In the end I was able to trace the issue back to the content types or mime-types that were being assigned to the files when they were being upload to Azure blob storage. I discovered that while videos with the mimetype of “video/mp4” played fine, other that had been labeled as “Video/mp4” would not play. The difference being the upper case V. Windows 8 running IE 10 and IE 11 seem to have no problem with this, but something in the Win7/IE10 combination has issues. (IE9 seemed to work fine). From what I could tell, according to RFC 2045 the standard that appears to document content types, case should not matter:
"The type, subtype, and parameter names are not case sensitive. For example, TEXT, Text, and TeXt are all equivalent top-level media types."
Oddly, in the end it did matter. Hopefully if you’re searching for Internet Explorer 10 HTML5 video playback issues on Windows 7 you’ll find this solution sooner than I did.