RobGMiller

Thanks for the reply but that made no difference. It should work without the codecs specification according to the w3schools test page which does work. If I use the <source tag without the end tag />, the Visual Studio error detector tells me "Validation (XHTML5): Empty elements such as 'source' must end with />" Could this be an issue?

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?