pos:abs + max-width + margin:auto

Block centered: Firefox, Safari, Chrome Aligned to left: Opera, IE

HTML

<div></div>

CSS

DIV {
    position:absolute;
    top:0;
    right:0;
    bottom:0;
    left:0;
    margin:0 auto;
    width:200px;
    background:#C00;
    }

JavaScript

var div = document.querySelector('div').onclick = function() {
    this.style.width = 'auto';
    this.style.maxWidth = '200px';
}