Safari 11.0 on High Sierra does not play any youtube.com videos. It displays instead:
Your browser does not currently recognize any of the video formats available. Click here to visit our frequently asked questions about HTML5 video.
Youtube videos work just fine on Chrome, but just avoiding advertising bombardment has been enough to become a fan of Safari (even though it's particularly slow at reading my brief start-up HTML file).
How do I add youtube support to Safari 11.0?