Custom cleanup

Source

Below is all you need to setup the example.

<script type="text/javascript" src="<your installation path>/tiny_mce/tiny_mce.js"></script>
<script type="text/javascript">
tinyMCE.init({
	mode : "textareas",
	theme : "advanced",
	setup : function(ed) {
		// Gets executed before DOM to HTML string serialization
		ed.onPreProcess.add(function(ed, o) {
			// State get is set when contents is extracted from editor
			if (o.get) {
				// Add span element to each strong/b element
				tinymce.each(ed.dom.select('strong,b', o.node), function(n) {
					n.appendChild(ed.dom.create('span', {style : 'border: 1px solid green'}, 'Content.'));
				});
			}
		});

		// Gets executed after DOM to HTML string serialization
		ed.onPostProcess.add(function(ed, o) {
			// State get is set when contents is extracted from editor
			if (o.get) {
				// Replace all strong/b elements with em elements
				o.content = o.content.replace(/<(strong|b)([^>]*)>/g, '');
				o.content = o.content.replace(/<\/(strong|b)>/g, '');
			}
		});
	}
});
</script>

<form method="post" action="somepage">
	<textarea name="content" style="width:100%">
	</textarea>
</form>