This document discusses how CSS (Cascading Style Sheets) is used to style and format HTML documents. CSS describes how HTML elements should be presented and is kept in a separate file from HTML code. CSS allows formatting of elements like comment boxes, background colors, fonts, and navigation bars. The document explains how to link a CSS file to an HTML file using the <link> tag and provides an example of CSS code to change the font color of <h1> tags to blue.