<html>
<head>
<style>
.floatblock {
  overflow:hidden;
  float:left;
  background-color:black;
  position:relative;
  margin:5px;
  right:50%;
}
</style>
</head>
<body>
<div>
The six boxes below should be centered in two rows of three.
</div>
<div style="float:left; position:relative; left:50%">
<div class="floatblock"><img width=200 height=200></div>
<div class="floatblock"><img width=200 height=200></div>
<div class="floatblock"><img width=200 height=200></div>
<br>
<div class="floatblock"><img width=200 height=200></div>
<div class="floatblock"><img width=200 height=200></div>
<div class="floatblock"><img width=200 height=200></div>
</div>