I wrote a quick bookmarklet for you:

javascript: (() => document.querySelectorAll("video").forEach(video => video.loop = true))();

You can paste this into the URL bar (and press Enter to apply) or add it as a bookmark (and apply by clicking on the bookmark), it will make all video elements on the page loop automatically. Caveats:

• It won’t work if the video element is in a subframe (shouldn’t be the case too often with normal pages)
• Only works for HTML5 video elements
• Custom controls might override the native loop, but they shouldn’t