Unique identifier of an element; allows element to be cross-referenced. Value must be unique across a document, not just for an element type.
The ID and IDREF attribute values constitute a primitive reference mechanism for XML. For example, when an @id attribute has been used on a <glyph-data> element, that identifier can be referenced by an IDREF-style attribute on a <glyph-ref> element, so that the full glyph data can be pointed to and need not be repeated every time the character is used.
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.)
| Value | Meaning |
|---|---|
| An identifier (ID) | A unique identifier for the element. |
| Restriction: This attribute must be specified if the element is used. | |
| Value | Meaning |
|---|---|
| An identifier (ID) | A unique identifier for the element. |
| Restriction: This attribute may be specified if the element is used. | |