The Rules page is where to point people when things are going wrong on the road to banning. The guidelines are where to point people when they're not quite sure about how to use #css specifically or IRC in general.

Breaking the rules is bannable. Breaking the guidelines is not. Continually breaking guidelines and being disruptive about it is breaking a rule, but that's not inherent to guidelines.

The "What is on topic" is more of a guideline, than a rule.