I’ve only played with this spec briefly in Safari’s “experimental” implementation of it, but was disappointed to see that the very thing I wanted to do most with it didn’t seem to be possible. Specifically, overlapping background images when one is set to repeat along both axes as a pattern. The image that is set to repeat and fill the entire area works just as well as always, but any attempts to overlay additional, non-repeating images on top of that pattern fail, as they simply never seem to draw. Whether this is an issue with the specification or Safari/WebKit’s implementation of it, I am not sure, but it’s something that should be fairly simple to implement and mandate.