Per Andrew Nacin's post this morning I've been testing TinyMCE trying to break it. I've found that when trying out various combinations, a nested tag inside a parent with attributes will be removed if it is empty, even if it has attributes of it’s own. I’ve tried this with just about every tag I could think of that would potentially be an empty element, i.e. divs, spans, i’s (for icon fonts), anchors all of which could conceivably be an element element and styled with a background image/font via css. Inputting the element via the Text tab or via insertion via TinyMCE plugin, and then switching to the Visual tab and back, the element is removed.