Today, I couldn't watch a video embedded on movies.stackexchange.com on my phone. Then, doing a search I found that those questions exists:
Why are you forcing the flash player?
Any embedded video gets automatically this code on Stack Exchange:
<embed src="http://www.youtube.com/v/hFWBJgirKfw" type="application/x-shockwave-flash" >
When you add this to your code
type="application/x-shockwave-flash", you force the YouTube flash player. All you have to do is this:
<iframe width="560" height="315" src="https://www.youtube.com/embed/hFWBJgirKfw" frameborder="0" allowfullscreen></iframe>
Embedding videos is not rocket science. Let YouTube decide when to show the HTML5 Player (if the user is on mobile) or the Flash Player (if the browser doesn't support HTML5); Google knows how to take care of that. Otherwise, you're preventing mobile users from watching any embedded videos on their phones, because most modern phones explicitly do not include Flash in their software anymore and it is not available on their stores. You're also punching people like me, who love the YouTube HTML5 player, in the face.
Flash Player is dead. Its time has passed. It's buggy. It crashes a lot. It requires constant security updates. It doesn't work on most mobile devices. It's a fossil, left over from the era of closed standards and unilateral corporate control of web technology. Websites that rely on Flash present a completely inconsistent (and often unusable) experience for fast-growing percentage of the users who don't use a desktop browser. It introduces some scary security and privacy issues by way of Flash cookies.
Flash makes the web less accessible. At this point, it's holding back the web.
Apple.com also spoke against the use of Flash in a post here:
Flash was created during the PC era – for PCs and mice. Flash is a successful business for Adobe, and we can understand why they want to push it beyond PCs. But the mobile era is about low power devices, touch interfaces and open web standards – all areas where Flash falls short.
New open standards created in the mobile era, such as HTML5, will win on mobile devices (and PCs too). Perhaps Adobe should focus more on creating great HTML5 tools for the future, and less on criticizing Apple for leaving the past behind.
Stop forcing flash.