CSS

====Selector types====
Selectors may apply to the following:
*all [[HTML element|elements]] of a specific type, e.g. the second-level headers [[HTML element#Basic text|h2]]
*elements specified by [[HTML attribute|attribute]], in particular:
**''id'': an identifier unique within the document, denoted in the selector language by a [[number sign|hash]] prefix e.g. {{Code|code=#id}}
**''class'': an identifier that can annotate multiple elements in a document, denoted by a [[period (punctuation)|dot]] prefix e.g. {{Code|code=.classname}} (the phrase "CSS class", although sometimes used, is a misnomer, as element classes—specified with the [[class attribute (HTML)|HTML class attribute]]—is a markup feature that is distinct from browsers' CSS subsystem and the related W3C/WHATWG standards work on document styles; see [[Resource Description Framework|RDF]] and [[microformat]]s for the origins of the "class" system of the Web content model)
*elements depending on how they are placed relative to others in the [[Document Object Model|document tree]].

Classes and IDs are case-sensitive, start with letters, and can include alphanumeric characters, hyphens, and underscores. A class may apply to any number of instances of any element. An ID may only be applied to a single element.