A selector is a pattern; it’s the part of a CSS rule that matches a set of elements in an HTML or XML document. The declarations that appear in the block that follows the selector are applied to all elements that match this pattern, unless they’re overridden by another rule in the cascade.

As is discussed briefly in Selectors, a selector can contain a chain of one or more simple selectors separated by combinators. A pseudo-element—for example, :first-line—can also be included after the last simple selector in the chain.

A simple selector contains either an element type selector, such as h1, or the universal selector, *. The universal selector can be considered to be implied (and can therefore be omitted) if it isn’t the only component of the simple selector.

A simple selector can also contain class selectors—for example, .warning, ID selectors—for example, #menu, attribute selectors—for example, [type="submit"], and pseudo-classes—for example, :hover. These act like modifiers on a type selector (or the universal selector), and qualify the selector, as if to say “but only if …”