I've inherited a theme from a another developer that I've re-styled for a client. We're just about finished, but I thought I'd run it through the Theme-Check plug-in to see if there's any errors or problems.
Unfortunately there's a ton. I've gotten rid of most of them, but I have one Required left:
REQUIRED: Could not find post_class. See: post_class
I've read the WP page on post_class, but as a relative newcomer I still don't understand what it is, or why it should be required.
What should I do to fix this?