Adding of HTML Elements

* HTML_ELEMENT: Represents a core HTML <element> for all tags.
* HTML_DATA: Start of a table cell data <td></td>
* HTML_ELEMENT_HELPER: Delegate helper functions to HTML_ELEMENT et al
* HTML_FACTORY: Given HTML_ELEMENT_HELPER instead of HTML_ELEMENT.
* HTML_TABLE: Beginnings of a <table></table> element.
* HTML_ELEMENT_TEST_SET: Tests of various forms of HTML_ELEMENT.