https://18f.gsa.gov/2015/09/28/web-design-standards/
https://github.com/18F/web-design-standards
Very nice; this is especially important for government sites. It's even printer-friendly out of the box!
http://assets.cms.gov/resources/framework/3.0/Pages/#documen...
?
(Which seems dormant/dead unfortunately).
https://18f.gsa.gov/2015/09/28/web-design-standards/
https://github.com/18F/web-design-standards