try validating:

  tag1 {
     box-shadow: inset 10px 10px;
  }

  tag2 {
     box-shadow: 10px 10px;
  }

as css3 - first one passes, second one returns:

  Parse Error [empty string]

(see http://www.w3.org/TR/css3-background/#the-box-shadow for reference).