Using all four positioning parameters with position:absolute - is this acceptable?

I provided this idea for providing overlay CSS, as an answer to another question. I hadn't thought of using this type of syntax in the past but I can't think of any problems that might be associated with using it.

As far as I can tell this works - and if admissible I think it provides an innovative solution - but I don't see it used often.

Could someone explain to me why it might be bad?

.ui-widget-overlay { 
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;

  background-color: #444;

  /* add some opacity here */
}