Not a polyfill, but does the trick for graceful degradation without complex and buggy JavaScript code.

ie8placeholders.js

/*jslint browser: true, plusplus: true, todo: true, white: true */
/*globals Drupal, jQuery */

// Simple placeholder fix for IE8, not a polyfill:
// If a field has no label displayed:
//  - If the label is hidden (sr-only or element-invisible) then just show it.
//  - If there is no label but a placeholder, build a label using the
//    placeholder value.
(function(jQuery) {
  "use strict";
  Drupal.behaviors.ie8placeholder = {
    attach: function (context) {
      var jContext = jQuery(context);
      jContext.find('[placeholder]').once('ie8placeholder').each(function () {
        var label;
        if (this.id) {
          label = jContext.find('label[for=' + this.id + ']');
          if (label.length) {
            // Element has a label
            label.removeClass('element-invisible').removeClass('sr-only').show();
          } else {
            // Element has no label
            jQuery(this).before('<label for="' + this.id + '">' + this.placeholder + '</label>');
          }
        }
      });
    }
  };
}(jQuery));