All XHTML Document can have only one root element that is <html>all other elements should be nested within the root element. There will be only one parent called a root element which can have its children(sub elements) nested within it.
Sub elements must be in pairs and correctly nested within their parent element. The basic document structure is:
Element ids which should be uniqe eventually will replace element names.For now, to ensure backword-compatibility it is recommended to use both,for example, many of the form fields are accessed by their names. In cascading style sheets and various scripting languages Ids are used .