With the release of Safari 11.1 on macOS and Safari on iOS 11.3, developers now have the ability to support background videos (mp4 support only - at the time of this article) with pure CSS.

Example:

.video-background {
    background-image: url('path-to-video.mp4);
}

See it in action

Check out the very basic CodePen I created below to see it live (make sure you view it in latest Safari or else you won't see anything :P)

CSS Video Background (Safari Only)

You can read up on all the new features implemented in 11.1 Safari here:

New WebKit Features in Safari 11.1