Which of the following is part of CSS structure?

CSS structure is fundamentally built around selectors and declarations. Selectors are patterns used to select the elements you want to style in the HTML document. They can target specific elements based on types, classes, IDs, attributes, or their relationship to other elements in the document. Declarations, on the other hand, define the styles that will be applied to the selected elements, consisting of property-value pairs.

For example, in the rule h1 { color: blue; }, h1 is the selector that targets all <h1> HTML elements, and color: blue; is the declaration that sets the text color of those elements to blue. This combination of selectors and declarations is what allows CSS to control the appearance and layout of a webpage.

The other options do not directly pertain to the structure of CSS. Character sets relate more to the encoding of text on a web page, web browsers are software applications that render web pages, and HTML tags are part of the HTML structure itself, rather than the styling language of CSS.