I have a critique, which is your naming convention for classes. This sentence from your site highlights the issue:

'Consider pure-g to be a row or "grid", and pure-u-- to be columns, or "units"'

First, I'm not a fan of the "pure-" prefix. If I were to use your system I'd use it exclusively. An option to either specify or ditch the prefixes would be lovely.

Second, in that sentence you say "consider pure-g to be a row". Why not just call it a row? I understand Bootstrap has used that already, but there's no copyright and it's a convention everyone is used to.