Something you might also be interested in is ‘document.designMode‘.
💬 Browser Inspector as the New Goggles
My suggested replacement is not quite equivalent in ease of use and share-ability, but at the same time it opens the door to a powerful tool for not only understanding what lives below the web page, but tinkering with it learn CSS modification trickery.
Thank you Alan for sharing your write-up. It is kind of sad to see X-Ray and Thimble go, but at least there are alternatives.