Assuming the element which is floated and will be centered is a div
with an id="content"
…
<body>
<div id="wrap">
<div id="content">
This will be centered
</div>
</div>
</body>
And apply the following CSS:
#wrap {
float: left;
position: relative;
left: 50%;
}
#content {
float: left;
position: relative;
left: -50%;
}
Here is a good reference regarding that.