Safari 9.0 (and above) exposes an API to allow HTML5 audio and video to be streamed via AirPlay. More info on this is available here and here.
Safari 10.0 (and Safari Technology Preview) exposes an API to allow HTML5 video to participate in macOS (and iOS)'s Picture-in-Picture mode, where the video is popped out of the browser and floats above other content on the screen. More info on this is available here and here (implementation is the same for iOS and macOS).
This seems like something that could be implemented very easily and would benefit users of Safari greatly.
Vimeo has already implemented support for both of these features and both can be seen on any video on their site when using Safari 10 or Safari Technology Preview. (Picture-in-Picture requires macOS 10.12.) Here is a link to a Vimeo video for your convenience.
I am interested in helping beta-test these features. Thank you for your time!