var gMinLeftMargin = 0;

window.onload = function () {
  itpCentreLayers();
  onresize = itpCentreLayers;
}

function itpCentreLayers()
{
  itpCentreTheLayer('top');
  itpCentreTheLayer('header');
  itpCentreTheLayer('img-01');
  itpCentreTheLayer('main');
  itpCentreTheLayer('footer');
}

function itpCentreTheLayer(ALayer)
{
  var i, winWidth, layerObj, x;
  winWidth = (document.all) ? document.body.clientWidth : window.innerWidth;

  if (layerObj = document.getElementById(ALayer)){
    x = (winWidth - layerObj.offsetWidth + gMinLeftMargin) / 2;
    if (x < gMinLeftMargin) x = gMinLeftMargin;
    layerObj.style.left = x;
  };

}