2.2.4 [DOM Level 2 - HTML] Section 1.6.3, Exposing Element Type Names
If the document is an HTML 4.01 document the element type names exposed through a property are in uppercase. For example, the body element type name is exposed through the tagName property as BODY. If the document is an XHTML 1.0 document the element name is exposed as it is written in the XHTML file. This means that the element type names are exposed in lowercase for XHTML documents since the XHTML 1.0 DTDs defines element type names as lowercase, and XHTML, being derived from XML, is case sensitive.
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XHTML is not supported.