When you have that luxury. There's flexbox too, but that's not broadly supported at the time of this writing.
HTML:
<div class="content">This works with any content</div>
CSS:
.content {
position: absolute;
left: 50%;
top: 50%;
-webkit-transform: translate(-50%, -50%);
transform: translate(-50%, -50%);
}
Tinker with it further on Codepen or on JSBin
For older browser support, look elsewhere in this thread.