a.annotation {
  position: absolute;
  border: 3px solid blue;
  padding: 0;
  display: none;
}
a.annotation span {
  display: block;
  background: blue;
  width: 100%;
  height: 100%;
  opacity: 0.2;
  -moz-opacity: 0.2;
  filter:alpha(opacity=20);
}
a.annotation:hover {
  border-color: red;
}
img.annotated {
  border-width: 0;
}