The document introduces CSS (Cascading Style Sheets) and the box model. CSS allows applying rules to elements on a web page to control how they are rendered. Rules have selectors that indicate elements to style and declarations that set properties. CSS rules can be defined in external style sheets, internal style sheets within <style> tags, or inline with the style attribute. The box model conceptualizes elements as boxes that may have borders, margins, padding, and dimensions. Attributes of these boxes like color, width, and overflow can be controlled with CSS.