From http://student.wordflyers.com/#/learn/grade_levels Get the maximum z-index (or potentially any style attribute on a page (uses jQuery)

new_gist_file_0

(Math.max.apply(Math, 
  $.makeArray($('*')
  .map(function () {
    return $(this).css('z-index');
  }).filter(function () {
      return $.isNumeric(this);
  }).map(function () {
      return parseInt(this, 10);
  }))));