a way of applying CSS to something not directly present in the markup structure of a document, like the first line of text in a paragraph or a form control that is currently "valid
pseudo-classes are used in CSS selectors to allow information external to the HTML source (e.g. the fact that an anchor has been visited or not) to classify elements.
A means of selecting certain text in an HTML document, based not on the HTML code of the document itself, but on other external conditions applied by the web browser.