Added the following HTML5 Video tag to an aspx page. The tag is not recognized on this page in IE11 but it works as expected with chrome.
<video width="400" height="300" controls="controls">
<source src="/InstructionVideo.mp4" type='video/mp4;codecs="avc1.4D401E, mp4a.40.2"' />
Your browser does not support the video tag.
</video>
The W3Schools Video (Try it yourself) page works as expected in IE11.
If this markup is added to a simple html file it works as expected in IE11.
This application page contains a lot of other markup and javascript, including a fair amount of dynamic page element manipulation in the Page_Load.
In addition, if the simple html file is launched from the aspx file using <a href="/InstructionVideo.html"> it doesn't work.
If the simple html file is launched in a different window from the aspx file using <a onClick="window.open('InstructionVideo.html')"> it doesn't work.
So, the question is; What kind of code can be present on an aspx page that might prevent the HTML5 Video tag from rendering properly?