I just have a try, new a new WIN8 App JS blank project. and Just change default page to add Youtube embed content in, like following one
<iframe width="420" height="315" src="//www.youtube.com/embed/AxTiaInrLW4?rel=0" frameborder="0" allowfullscreen></iframe>
It can't work in my WIN8 App, but it does work for normal HTML, and open in a browser.
Have someone know the issue? and How to resolve it?