Simple HTML page with some JavaScript to loop YouTube videos since none of the sites that I've used to do this are very pleasant to use.
A live version of the page can be found at https://www.youtubeencore.com/
Adding ?v=[video id] after the URL will load a video. /watch?v=[...] also works to make it easy to use straight from YouTube by adding "encore" to the URL, e.g. https://www.youtubeencore.com/watch?v=WBAIyY9y3HM