macOS Sierra brings picture-in-picture videos. In Safari, Right–click (or Control–click) the video and choose Enter Picture-in-Picture. The video will change to a small floating window that you can position in any corner of the screen.
macOS Sierra brings system-wide access to tabs for many Apple apps as well as third party apps. Apps may need to be updated to take advantage of them, but Apple has updated many of its own apps (such as Maps, Keynote, Numbers, etc.) to support tabs.