The getElementsByClassName() is already present in some modern browsers, but this code from Kevin Yank at Sitepoint just adds it for those browsers that don’t already have it….

function getElementsByClassName(className) {
     // get all elements in the document
     if (document.all) {
         var allElements = document.all;
     }
     else {
         var allElements = document.getElementsByTagName("*");
         }

     var foundElements = [];

     for (var i = 0, ii = allElements.length; i < ii; i++) {
         if (allElements[i].className == className) {
             foundElements[foundElements.length] = allElements[i];
         }
     }

     return foundElements;
}