Mozilla's X-Ray Goggle's is a service that I've been promoting for years as way to help students understand how HTML works. Students can use it to view and change the HTML behind almost any webpage that they find. The changes happen as a local copy of the page that students could then share with their teachers.
Unfortunately, Mozilla has announced that they are discontinuing support of X-Ray Goggles. On December 16, 2019 Mozilla will close X-Ray Goggles and delete all user data associated with the project.
At the end of their announcement Mozilla recommended looking at Glitch as a possible alternative to X-Ray Goggles. I'd also recommend taking a look at Tynker.