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.
Did you know in Apple’s Mobile Safari browser you can reopen a tab that you recently closed? It’s pretty well hidden, so many people would never know. It remembers the last 5 tabs you closed, and here’s how to find it.
In Mac OS X Yosemite, you can preview links in Safari with a 3-finger tap. Hover over a link in Safari, and tap your trackpad with 3-fingers to see a preview.