This document discusses using vertical rhythm and modular scale principles in web design with Sass. It describes how vertical rhythm creates a consistent spacing relationship between stacked elements, and how a modular scale uses a common ratio to relate numeric values. The document recommends using these typographic rules with Sass functions and math to create a consistent user interface. It provides examples of how to set up and use a Sass toolkit called Typesettings to implement vertical rhythm and modular scale in a project.