ID: -- Unique identifier. For example, most elements have an ID attribute, so that a unique code can be established for and used to refer to that specific element. The content of the ID attribute is of the type called "id." Parsers verify that the value of attributes of type "id" are unique. ID attribute values must begin with an alpha, not numeric, character, either upper or lowercase, and may contain a . (period), : (colon), - (hyphen), or _ (underscore), but not a blank space. See also attributes of type "idref."