http://ez.no/doc/ez_publish/technical_manual/3_6/reference/xml_tags lists the current available default xml-tags to use in xml-fields. Take a look at the literal-tag. HTML-tags are not directly supported.
It is also possible to make your own xml-tags. The way xml-tags are presented in attribute_view_gui is template driven.
In the beginning things like this drove me crazy, but now i see it as the beauty of eZ :-) Good luck!