What DomesticExtremis posted is an image acting as a link, not a embedded video like Youtube allows as you mentioned in the last part of your post.
If you want to have an image as a link that will open the reader's browser and play a YouTube video you can do that simply by doing what DomesticExtremis said. That is supported both in ePub2 and ePub3 versions since it's just a link.
If you want to embed a YouTube video into an ePub you need to go to YouTube and use the embed code as is and you will get an embedded video, please note this will only work in ePub3 version and not all devices support this feature yet.
In addition, you need to add a property in your metadata.opf to that html file. What I mean!?
If you have an html file with the video called outvideo.html for example, this is the code you should insert in order to have an embedded video:
Code:
<iframe width="560" height="315" src="//www.youtube.com/embed/-V4JYIOja10" frameborder="0" allowfullscreen></iframe>
Code:
<object width="560" height="315"><param name="movie" value="//www.youtube.com/v/-V4JYIOja10?version=3&hl=en_US"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="//www.youtube.com/v/-V4JYIOja10?version=3&hl=en_US" type="application/x-shockwave-flash" width="560" height="315" allowscriptaccess="always" allowfullscreen="true"></embed></object>
Can't remember which one works, if both works use the old embed code (<object> tag)
Your changes to the manifest in the metadata.opf:
Code:
<item id="outvideo" href="outvideo.xhtml" media-type="application/xhtml+xml" properties="remote-resources" />