Why doesn't the HTML filter work if you select one group?
From Joomla! Documentation
In version 1.5.8 and 1.5.9, there is a bug in the Article Manager → Parameters → Filtering Options. If you select just one group to be filtered, no filtering is applied to any group. The work-around for this bug is very simple -- just be sure to select two or more Filter groups. For example, if you just want the Author group to be filtered, select Author and Registered and it will work correctly. Since Registered users cannot normally submit articles, this will have no effect other than to make sure the Author filtering works correctly. This bug will be fixed in version 1.5.10.