I did have to include an allowfullscreen="false" attribute to the iframe in order to get this to work. I made a one page epub3 with sigil. Then I unzipped the epub and manually edited the "preface.xhtml" page. Then re-zipped it and uploaded it to my website.
I write and sell online how-to-build-boats ebooks. I sell a password to a website version of the book, with numbered pages etc. I'm working on a python utility that runs from cron, that makes a downloadable epub every night........for those who purchased a password.
This may not (probably will not) work in many if not most readers. But it does work in Chrome and Brave. I haven't tired Firefox yet.
So many authors and readers want embedded video it seems inevitable wider support will gradually happen. In other words support is minimal now but that will change over time.
Complaints about "link rot" to youtube are common. Solutions will evolve.
https://montana-riverboats.com/epubit.epub
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>epub mess</title>
</head>
<body>
<p>sample epubit.epub with iframed youtube video</p>
<iframe width="560" height="315" src="https://www.youtube.com/embed/YB6gwOBClwE" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="false"></iframe>
</body>
</html>