The RunSignUp Widget now works with Responsive Design websites. The planned new Widget 2.0 release next week will make some of this easier, but the technical documentation is below for completeness:
If clients are using a responsive template, they will have to use different versions of the embed code surrounded by media queries (e.g. ‘visible-phone’, ‘visible-desktop’, etc.) as it only responds to the custom width set by the widget. If a width is not set, it goes to the default 600px.