[pornel]

Sizes of "Retina" PNGs can be halved just by using better PNG compressors, here's real-world example:

http://imageoptim.com/tweetbot.html

It's annoying that we have the tools. We don't need new format for this, just use what we have already!

[ronaldj]

But you won't get the file size of a lossy WebP image with an alpha channel.