DTD Attribute Types - ID
The attribute type of
ID is used specifically to identify elements.
Because of this, no two elements can contain the same value for attributes of type
ID. Also, you can only give an element one attribute of type
ID. The value that is assigned to an attribute of type
ID must be a valid XML name.
Valid XML - The following XML document would be valid, as it conforms to the above DTD:
Invalid XML - The following XML document would be invalid because the value of the
mountain_id attribute is the same for both elements: