Thanks that's an interesting trick.

This is beautified if somebody wants to see how it is done.

  function() {
    var myBody = document.getElementsByTagName('body')[0];
    var myBodyWidth = myBody.style.width;
    if (!myBodyWidth || myBodyWidth === 'auto' || myBodyWidth === 'inherit') {
        myBody.style.width = '1200px';
        myBody.style.marginLeft = 'auto';
        myBody.style.marginRight = 'auto';
        myBody.style.position = 'relative';
        myBody.style.cssFloat = 'none';
    } else {
        myBody.style.width = 'auto';
        myBody.style.position = 'static';
    }
  }