Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag. Each element has a type, identified by name, sometimes called its "generic identifier" (GI), and may have a set of attribute specifications. Each attribute specification has a name and a value.
 element ::= EmptyElemTag | STag content ETag [ WFC: Element Type Match ] [ VC: Element Valid]
This specification does not constrain the semantics, use, or (beyond syntax) names of the element types and attributes, except that names beginning with a match to (('X'|'x')('M'|'m') ('L'|'l')) are reserved for standardization in this or future versions ...