I was helping a friend with a problem and thought I share my solution with you.
If you add a video embed and add corners to that div it will not work in safari.
The solution is a little extra CSS.
First make div with corner radius and and set it to overflow hidden, and also a width. Put the video element inside that div.
Now that wrapper div needs some extra css
-webkit-mask-image: -webkit-radial-gradient(white, black);
You can see a working example here: https://vimeo-test.webflow.io
Hope it is helpful for someone.