Returns the Element that has an ID attribute with the given value. If no
such element exists, this returns null. If more than one element has an
ID attribute with that value, what is returned is undefined.
The DOM implementation is expected to use the attribute Attr.isId to
determine if an attribute is of type ID.
Note: Attributes with the name "ID" or "id" are not of type ID unless so defined.
Returns the Element that has an ID attribute with the given value. If no such element exists, this returns null. If more than one element has an ID attribute with that value, what is returned is undefined. The DOM implementation is expected to use the attribute Attr.isId to determine if an attribute is of type ID.
Note: Attributes with the name "ID" or "id" are not of type ID unless so defined.