I really want a better alpha-mask standard to emerge. PNG24 is attractive, but huge. PNG8/GIF works well, but is pretty limited. I don't think WebP is it, but we'll see what comes out of the scrum. I'll work with whatever that is, but I won't waste my time chasing will o' the wisp "standards."
Some time ago I experimented with a reduced color palette and dithering while keeping the fill alpha info. It reduced the file size a lot while keeping the alpha.
I started writing Web sites in the mid-'90s, where a page was supposed to be about 30K (quaint, huh?).
The prevailing wisdom, then, was no dither, adaptive palette, and reduce the color palette until it hurts, then back up one.