function ZIPFocus() {
  ref = document.getElementById('zip');
  if (ref.value == 'Enter a ZIP code')
    ref.value = '';
}

function ZIPBlur() {
  ref = document.getElementById('zip');
  if (ref.value == '')
    ref.value = 'Enter a ZIP code';
}

function onWindowLoad() {
  document.getElementById('zip').onfocus = ZIPFocus;
  document.getElementById('zip').onblur = ZIPBlur;
}

window.onload = onWindowLoad;