The standard approach is to give the centered element fixed dimensions, and place it absolutely:
<div class='fullscreenDiv'>
<div class="center">Hello World</div>
</div>?
.center {
position: absolute;
width: 100px;
height: 50px;
top: 50%;
left: 50%;
margin-left: -50px; /* margin is -0.5 * dimension */
margin-top: -25px;
}?