Re: WebP, anyone using it?

IMHO it is worth having a use case for multiple image formats. While they may not be prevalent in the wild, I don't think we should discard the ability to use them.

 I agree with the following markup pattern:
<picture>
    <source type="image/webp" srcset="small.wp, medium.wp 2x, big.wp 3x">
    <source srcset="small.gif, medium.png, big.jpg">
</picture>