What is the term for a set of CSS rules applied to HTML elements?

The term that describes a set of CSS rules applied to HTML elements is commonly referred to as a style sheet. A style sheet encompasses all the necessary rules and declarations to define how HTML elements should be displayed on a web page.

Each style rule within the style sheet consists of selectors and declarations that specify the properties and values for the HTML elements. For instance, a style sheet can dictate font sizes, colors, spacing, and layout for a website.

It's important to distinguish that the other terms, while related to CSS, do not encapsulate the full scope of what is being asked. A declaration refers specifically to a single property-value pair within a rule, a rule itself pertains to the entire combination of a selector and its declaration block, and a property indicates a specific CSS attribute within that declaration. Hence, the correct answer effectively captures the concept of a collection of CSS rules working together, which is fundamental for applying styles to web documents.