How to make reponsive CSS with ocsigen?

The best way to do that is to make one general css sheet plus three css sheets, small, medium and large screen using media queries, a feature introduced in CSS3.

Write theses lines in your css sheets:

@media only screen and (max-device-width: 480px)
@media only screen and (min-device-width: 481px) and (max-device-width: 768px)
@media only screen and (min-device-width: 769px)

You can find more informations about making responsive websites with media queries on W3schools.com