Site is broken when viewed in Safari

My website is sometimes completely broken in Safari and sometimes only a little bit. I didn’t ran into any problems with other browsers. I mainly tested and designed it in Firefox on Mac and used a template.

I have attached a few Screenshots and also videos to show how it should look like, vs. how it looks in Safari. The big problem is that all the agencies I want to send out the website to will most likely use Mac and Safari.

Main issue I have is that the rounded edges on background videos aren’t shown properly and I didn’t figured out why and tried to solve it on my own, but I just don’t know why it won’t work.
On my friend’s iPhone it is even worse. The whole website is freezed somehow and none of the videos will play.

I thank everyone, who is trying to help me! Thank YOU!

How it should look like (Firefox):

How it should not look like (Safari on Mac):

How it absolutely shouldn’t look like (Safari on my friend’s iPhone):

Some screenshots:
Firefox on Mac:

Safari on Mac:

Safari on iPhone:

Here is my site Read-Only: [LINK][1]