Index: modules/filter/filter.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/filter/filter.module,v
retrieving revision 1.300
diff -u -p -r1.300 filter.module
--- modules/filter/filter.module	23 Oct 2009 22:24:14 -0000	1.300
+++ modules/filter/filter.module	30 Oct 2009 04:06:51 -0000
@@ -729,6 +729,18 @@ function _filter_tips($format_id, $long 
   foreach ($formats as $format) {
     $filters = filter_list_format($format->format);
     $tips[$format->name] = array();
+    // If the text format does not remove or escape any HTML, add a special
+    // filter tip which explains that HTML is allowed. We detect these cases by
+    // defining an invalid HTML tag and running it through the text format.
+    // Since any (reasonable) HTML filter uses a whitelist approach when
+    // filtering, we expect this tag to either be removed or escaped if the
+    // text format restricts HTML in any way.
+    $fake_tag = '<abcdefghij />';
+    if (strpos(check_markup($fake_tag, $format->format, '', TRUE), $fake_tag) !== FALSE) {
+      $tips[$format->name][] = array('tip' => t('All HTML tags are allowed.'), 'id' => 'filter-all-html');
+    }
+    // Now go through and add each of the regular filter tips associated with
+    // this text format.
     foreach ($filters as $name => $filter) {
       if (isset($filter_info[$name]['tips callback']) && function_exists($filter_info[$name]['tips callback'])) {
         $tip = $filter_info[$name]['tips callback']($filter, $format, $long);