like this
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>kicking my butt!</title>
<style>
* {
margin: 0;
padding: 0;
border: 0;
outline: 0;
font-size: 100%;
line-height: 1.5em;
text-decoration: none;}
header {
height: 400px;
text-align: center;
background:#000;
}
.header-middle h1 {
font-size: 16px;
line-height:400px;
font-family: Verdana, Geneva, sans-serif;
color: #0F0;
overflow:hidden;
float:right;
}
</style>
</head>
<body>
<header>
<div class="header-middle">
<h1>Text to be centered vertically within the black box, but don't know how...</h1>
</div><!--end of header-middle div-->
</header>
</body>
</html>