This text is set in JAF Bernina Sans, backed up by a Verdana stack. It uses a Sass map to scope font-size and line-height measurements to those specific typefaces, as Robin Rendle described. (Em units and max-widths, too.) See the variables and mixins in this other Pen.
Look at this Pen's own CSS and you'll see what I really wanted to explore. The paragraph element uses Verdana (and its associated measurements) by default, and only uses Bernina (and its associated measurements) if Bernina has loaded properly. The wf-active class is via Typekit font events. Progressive enhancement FTW.
When the composition widens and reaches a breakpoint, its size gets bumped up — again, considering Verdana by default and Bernina only if Bernina is present.
Tim B