I'm reminded here that JPEG includes arithmetic encoding as part of the standard, but almost everyone uses Huffman because up until a couple years ago arithmetic encoding was patent-encumbered (the patents are expired now). Is anyone aware of a study like Mozilla's that considers JPEG-with-arithmetic-encoding? Or perhaps it does, and I failed to notice?

Most competing file formats seem to beat JPEG by only a slim margin, and what I've read on arithmetic encoding suggests it gives a ~5-10% gain, which would make that difference slimmer still, perhaps vanishing into the uncertainty of the usefulness of these quality benchmarks. Of course, there would be inertia to overcome to support it, as with a new format, but recompiling everyone's libjpeg is surely less work than adding support for whole new file formats. At the very least, it seems there might be a better effort/payoff ratio.