Root element
（重定向自Document element）
Each XML document has exactly one single root element. It encloses all the other elements and is therefore the sole parent element to all the other elements. ROOT elements are also called PARENT elements.
The World Wide Web Consortium defines not only the specifications for XML itself, but also the DOM, which is a platform- and language-independent standard object model for representing XML documents. DOM Level 1 defines, for every XML document, an object representation of the document itself and an attribute or property on the document called documentElement. This property provides access to an object of type element which directly represents the root element of the document.