The Allowed HTML tags made available to content authors by default in the Limit allowed HTML tags input filter used in the default Filtered HTML text format is a bit too limited. For example, if we want to let people add images, headlines, or superscript text, we need to add the tags that make these possible: img, h1 through h6, and sup.
Here is an example of an expanded allowed tags list
<a> <em> <strong> <cite> <blockquote> <code> <ul> <ol> <li> <dl> <dt> <dd> <img> <h1> <h2> <h3> <h4> <h5> <h6> <tt> <output> <q> <sub> <sup>
TODO: Look into / mention security concerns with img tag?
TODO: Recommend against including h1?
TODO: Create a module or install profile to automatically expands the allowed tags list so that this need not be done manually on every site.