Make 0w invalid. Make sizes=0 round to a UA-specific minimum. Avoids possible discontinuities.
Merge branch 'zero-sizes' of github.com:ResponsiveImagesCG/picture-element into zero-sizes
remove ref to zero effective size
Allow source sizes to go to zero.
Bikeshed\!
Merge.
Move the srcset 0w banning to after the value is parsed.
Allow zero lengths in the sizes attribute again.
bikeshed
Merge pull request #151 from ResponsiveImagesCG/zero-sizes

Make 0w invalid. Make sizes=0 round to a UA-specific minimum.