The Interactive Advertising Bureau (IAB) sets guidelines for ad unit types for its members. When developing a website you’ll save yourself many headaches by playing by their rules. Without further ado, the z-index guidelines that the IAB sets as recommendations for page content and ad units: < 0 Background elements 0 - 4,999 Main content and ads that do not expand 5,000 - 1,999,999 Expanding ad un