Indicates the unique identifier for an associated element, so that the element’s content may be cross-referenced. IDs must be unique inside their document (not just within their element type); the XML parser enforces this.
The ID and IDREF attribute values constitute a primitive reference mechanism for XML. For example, when an id attribute has been used on a <fig> element, that identifier can be referenced by an IDREF-style attribute on a <xref> element, so that the figure can be pointed to and need not be repeated every time the figure is mentioned.
An identifier string is typically 1-32 characters in length and must start with a letter of the alphabet. (It is an XML rule that identifiers may not start with a digit.)
|An XML identifier (ID)||A unique identifier for the element. XML parsers can verify that an element pointing to one of these is pointing to a valid ID.|
|Restriction: This attribute may be specified if the element is used.|