HTML markup structure of an ad:
<div class="wpa [wpmrec|wpwidesky|wpleaderboard]">
<a class="wpa-about" href="http://wordpress.com/about-these-ads/" rel="nofollow">
overflow: hidden; /* this hides "about these ads" when there's no adfill */
max-width: 100%; /* important! this bit of CSS will *crop* any ad that's larger than the parent container! */
font: 10px/1 "Open Sans", Arial, sans-serif !important;
text-align: left !important;
text-decoration: none !important;
border-bottom: none !important; /* some themes ad dotted underlines, that won't look nice */
box-shadow: none !important;
.wpa .u > div { /* @todo: deprecate wpdvert */
margin-top: 5px; /* this makes "about these ads" visible */
margin-bottom: 1em; /* every ad should have a little space below it */
margin-top: 17px; /* this makes "about these ads" visible */
margin-bottom: 1em; /* every ad should have a little space below it */
background-color: transparent;