Find a page with a CSS problem (even bonus points if it's for work). Then, use a web debugger to identify and fix the problem (Firebug or Chrome). As someone

Find a page with a CSS problem (even bonus points if it's for work). Then, use a web debugger to identify and fix the problem (Firebug or Chrome). As someone else said, write your own responsive grid from scratch. Try writing a responsive contact form (name, E-Mail, message body).