Flexbox fluid grid, items on last row all split available space

Either add items to the list or change window width (or item width, or all of these) to see the effect. Flexbox does this out of the box. (Note Firefox has no flex-wrap support so it doesn't work there yet.)

See @heydonworks' article on accomplishing similar feats without Flexbox.

This demo brought to you by @stephenhay, who really, really likes Flexbox.