I have a working video embeded in sigil epub3 with this code
<div style="font-size: 16px;"><p style="padding-bottom: 3px; text-align: center; line-height: 1; margin: 0px;"><video src="../Video/class_sketch.mp4" width="432" height="292" poster="../Images/social-classes.jpg" controls=""></video></p>
The video plays in an epub reader but the thumbnail image does not show on the reader, yet it displays in the sigil page
I am using sigil 0.9