I’m genuinely disappointed that it’s not done with -moz-element() on Firefox, but still duplicates all the content and just scales it down. Using -moz-element() has the potentially-useful side-effect that things like text selection will be visible in the minimap as well, as well as avoiding the problems of the duplication approach, such as that browser find functionality looks in the minimap too. (The minimap needs `user-select: none` which fixes some of the problems, but not all.)
I still hope that other browsers will eventually implement element(), along with cross-fade() which is also in the CSS Images Module Level 4 draft <https://w3c.github.io/csswg-drafts/css-images-4/>.