1#parent {position: relative;}
2
3#child {
4    position: absolute;
5    top: 0;
6    bottom: 0;
7    left: 0;
8    right: 0;
9    width: 50%;
10    height: 30%;
11    margin: auto;
12}

1HTML:
2<div class="ext-box">
3	<div class="int-box">
4		<h2>Some txt</h2>
5		<p>bla bla bla</p>
6	</div>
7</div>
8
9CSS:
10div.ext-box { display: table; height: 100%; width:100%;}
11div.int-box { display: table-cell; vertical-align: middle; }