CSS positioning and window dimensions

Re: 'Positioning HTML Elements with Cascading Style Sheets' 

With this declaration:

   BODY { overflow: clip; }
   DIV  { top: 0; left: 0; width: 100%; height: 100%;
          overflow: scroll; }

and this markup:

   <P>Does this division element fill the UA window?

what happens?

If the default initial position and size of BODY could be considered
congruent with the UA's window, CSS positioning would provide an
alternative to frames for non-scrolling navigation bars.

David Perrell