CSS selectors – CSS: Cascading Style Sheets | MDN

A selector with a single component, such as a single id selector or type selector, that’s not used in combination with or contains any other selector component or combinator. A given element is said to match a simple selector when that simple selector accurately describes the element. All basic selectors, attributes, and single pseudo-classes and pseudo-elements are simple selectors.

A sequence of simple selectors that are not separated by a combinator. A compound selector represents a set of simultaneous conditions on a single element. A given element is said to match a compound selector when the element matches all the simple selectors in the compound selector.

In a compound selector, the type selector or a universal selector in a compound selector must come first in the sequence of selectors. Only one type selector or universal selector is allowed in the sequence. Since whitespace represents the descendant combinator, no whitespace is allowed between the simple selectors in a compound selector.

Example: a#selected {...}

A sequence of one or more simple and/or compound selectors that are separated by combinators. A complex selector represents a set of simultaneous conditions on a set of elements. These conditions apply in the context of relationships described by the combinators. A given element is said to match a complex selector when the element matches compound selectors and the combinators between the compound selectors.

Examples: a#selected > .icon {...}, .box h2 + p {...}, a .icon {...}

A selector representing an element relative to one or more anchor elements preceded by a combinator. Relative selectors that don’t begin with an explicit combinator have an implied descendant combinator.

Examples: + div#topic > #reference {...}, > .icon {...}, dt:has(+ img) ~ dd {...}

A comma-separated list of simple, compound, or complex selectors. If the constituent selector type of a selector list is important but unspecified, it is called a complex selector list. A given element is said to match a selector list when the element matches any (at least one) of the selectors in that selector list. Read more about when a selector list is deemed invalid and how to construct a forgiving selector list.

Example: #main, article.heading {...}