With HTML and cascading style sheets (CSS) mastered, this course features a comprehensive exploration of responsive design. Students explore Sass, a CSS preprocessing language that combines logic and variables with CSS to create dynamic styling. Students understand responsive design 2.0, combining Sass, the CSS data structures CSS Calc and CSS custom properties, plus Flexbox and Grid, to create new flexible layouts with less code. Students also examine a traditional responsive design framework incorporating Sass, like UIkit, and they compare and contrast the approaches in using an off-the-shelf responsive design framework as compared with a custom framework. The course culminates with students coding their own responsive design framework, including documentation and examples.