Cool guy, is an "open-source advocate" but removes PureCSS's copyright notice: https://github.com/yui/pure/blob/master/LICENSE.md
You are correct, that notice is not present, but I did not remove it intentionally. To be honest, I didn't read the LICENSE.md of Pure CSS. Shame on me :)
Feel free to file a bug against this at: https://github.com/ryanseys/ryanseys.com