You can clone with
webp has alpha now!
Looks like this shipped to Canary and will be available in Chrome 22.
I'd love weppy to manage this for me. AFAIK alpha webp images will just not load (instead of with a matte background) .. so the current onerror will trip for alpha ones, independent of regular non-alpha webp.
weppy could then supply the fallback in this case (see #1).
@paulirish webpjs seems to be a better solution than weppy, and does support transparency.
You might want to recommend it in html5please.