CSS: Absolute Center (Vertical & Horizontal) an Image

gistfile1

/*CSS background-image Technique*/
html {
   width:100%;
   height:100%;
   background:url(logo.png) center center no-repeat;
}
/*CSS + Inline Image Technique*/
img {
   position: absolute;
   top: 50%;
   left: 50%;
   width: 500px;
   height: 500px;
   margin-top: -250px; /* Half the height */
   margin-left: -250px; /* Half the width */
}