It's worth underlining the fact that "lossy" here means "really nicely dithered 256-RGBA-palette PNG8s", but (as the gallery examples demonstrate), that may be sufficient for most of the kinds of images that have significant transparency and that would be likely to be found on a webpage (even sprites for a canvas game).
Honestly, I would still like to see WebP support in spite of its drawbacks (thanks software patents!) for it's better definition of "lossy" and for saner alpha compression, but between better pngs from software like this and upcoming support for DXT5 in WebGL, most of the compelling market drivers may be gone.