What is the best modern way to do “Keep window on top” in OS X?

Marketing styles it as macOS nowadays, but anyway the modern way is when you are watching a video, click on the button that makes it pop out.

For example this clip on Vimeo https://vimeo.com/channels/staffpicks/361821058 in the bottom-right, one-to-last button is the popout button. The clip will now hover over all other windows. Other websites have other buttons, or have this in the generic right-click menu. And some simply don't offer this.

If you're asking for a way to keep any window on top in macOS, there isn't a way. The above also works in Safari, but not in Firefox.