[ldolse]

I played around on this a while back on my 505 and came up with this:

html:

  <div class="ruleclass">
    <hr />
  </div>

Stylesheet:

.ruleclass {
    margin-left: 5em;
    margin-right: 5em;
    }
hr {
   height: 3px;
   background:#505050;
   }

The hr styles are optional, the centering was accomplished by using the div. I'm sure there are alternate ways to do it, but I do recall arriving at that solution through trial and error, not sure what the specific failures were.