The CSS id selector ( X#id { } or #id { } ) is used to target a unique element in the document. Only one HTML element may be given a unique ID attribute value in your web document, so naturally this is meant for unique element targeting as opposed to grouped or class styling of elements. If the element type is not specified in front the "#" symbol, any type of element in the document given this ID attribute value will be styled. If the element type is specified only that type of element in the document can be styled, while other types of elements that may be given the unique id will get bypassed in styling. The ID selectors can be used in conjunction with other selectors to extend its reach into the document.

In the example below without the type specifier before the "#" symbol demonstrates how you can target any tag by the unique ID attribute value, regardless of element type. In the example above the element type must be "h3" in order to be styled.