This document discusses progressive prototyping as a design process. It emphasizes that prototyping allows designers to fail in a low-cost environment and iterate on their designs through testing. Different levels of prototyping fidelity are presented, from simple sketches to higher-fidelity visual prototypes. The document recommends starting with simple sketches and adding more details in subsequent rounds of graybox prototyping. It also provides examples of HTML5 and CSS3 techniques that can be used to create interactive prototypes, including new HTML5 input types, CSS3 selectors, and properties for styling buttons. Overall, the document promotes prototyping as a better way to validate designs through an iterative process of testing and refining ideas.