/* VERTICALLY */
div#splash {
width: 300px;
height: 200px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -100px;
margin-left: -150px;
}

/* HORIZONTALLY (1) */
body {text-align: center;}

div#content {
width: 40%;
margin: 0px auto;
text-align: left;
}

/* HORIZONTALLY (2) */
div#content {
position: absolute;
left: 50%;
width: 40%; /* could be in px as well */
margin-left: -20%;
}

Vertical & horizontal centering with CSS

CSS