body {
  width: 100%; height: 100%;
  margin: 0; padding: 0; 
  text-align: center; /* use text-align to make IE in quirks mode centre the divs */
  background: #fff;
  vertical-align: middle;
}
/* setup the width, margins and padding for all the divs */
#TopLayer, #flashcontent, #BottomLayer {
  width: 800px; /* see note below about widths */
  margin: 0 auto; padding: 0; /* margin 0 auto centres the divs horizontally for FF, Opera, IE in standards mode etc... */
}
/* take up half the height of the page with the topBar background */
#TopLayer { 
  height: 50%; 
}
/* and fill the other half with the bottomBar background */
#BottomLayer { 
  height: 50%;
}
/* now position your movie absolutely so it appears right in the centre */
#flashcontent { 
  position: absolute; 
  left: 50%; top: 50%; 
  margin-top: -300px;
  margin-left: -400px; /* again see note below */
}