I squash all my stylesheets into one giant .css file to serve rather than a bunch of individual stylesheets. A side-effect of squashing + minifying (which is done every time I generate my website) is that all comments are removed.

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