We support writing CSS with PostCSS here at CodePen, and as we just posted, we support a select number of PostCSS plugins. If you ever have a strong desire to see us support others, definitely let us know and we'll see what we can do.
But, you also don't need our help anymore! The ever-clever Jonathan Neal has figured out a way to use any PostCSS plugin you like here on CodePen, and it's perfectly good by us. It just takes a bit of work to make it happen.
The gist is that you use Browserify to build a script that bundles together the PostCSS plugin and processing with a little code to update the
<style> element in the Pen's preview area.