Markus Stange picked up the work I did in 2008 on the "-moz-element()" CSS extension (which was later extended by Ryo Kawaguchi), made some major improvements and got it reviewed and landed. Check out his blog post. -moz-element lets you render the contents of an element as the background image for any other element. This is a very powerful tool that can be used in very interesting ways; check out Markus' amazing demos. This feature is on trunk now and will be in Firefox 4. We will also propose this to the CSS WG.
Note for browser UI and extension authors: eventually -moz-element will be the preferred way to render "live" copies of Web page contents (insteading of using MozAfterPaint/drawWindow). Right now, -moz-element can be used to render the contents of a <browser> element elsewhere, although it's less well-tested and is less tweakable for performance. Post-FF4, we can tie -moz-element into the layers framework so that in many cases --- such as tab thumbnails --- rendering -moz-element just recomposites a layer subtree, fully GPU-accelerated.