I’m not sure what the exact use case was, but Sam Stephenson recently asked me: @chriscoyier What's the best way to reference the OS X system font in CSS (to get Lucida Grande or Helvetica, as appropriate)? — Sam Stephenson (@sstephenson) November 20, 2014 My immediate thought was to use the old trick where you put the User Agent in a data-attribute on the root element that you can select off of.