nts and attributes will be returned in lower-case.
Also, some XHTML elements may or may
not appear
in the object tree because they are optional in the content model
(e.g. the <code>tbody</code> element within
<code>table</code>).  This occurs because in HTML 4.0 some elements were
permitted to be minimized such that their start and end tags are both omitted
(an SGML feature).
This is not possible in XML. Rather than require document authors to insert
extraneous elements, XHTML has made the elements optional