In an HTML document, CSS ID selectors match an element based on the contents of that element's id attribute, which must match exactly the value given in the selector. The selector is written by first using the node name of the element, followed by a hash sign (#), then the name of the id. No white spaces are allowed between those three elements. For example, a with an id of id1 would be written as span#id1. The selector may also be written to identify only an id, not an id which matches the correct element. Example: #id1.