This CSS Reference shows the basic syntax of a CSS rule; lists all standard CSS properties, pseudo-classes and pseudo-elements, @-rules, units, and selectors, all together in alphabetical order, as well as just the selectors by type; and allows you to quickly access detailed information for each of them.
In an HTML document, CSS class selectors match an element based on the contents of the element's class attribute. The class attribute is defined as a space-separated list of items, and one of those items must match exactly the class name given in the selector.
A CSS pseudo-class is a keyword added to selectors that specifies a special state of the element to be selected. For example :hover will apply a style when the user hovers over the element specified by the selector.
Just like pseudo-classes, pseudo-elements are added to selectors but instead of describing a special state, they allow you to style certain parts of a document. For example, the ::first-line pseudo-element targets only the first line of an element specified by the selector.