getElementsByClassName is defined in HTML5. From WhatWG:
The getElementsByClassName(classNames) method takes a string that contains a set of space-separated tokens representing classes.
Given the following XHTML fragment:
<div id="example"> <p id="p1"/> <p id="p2"/> <p id="p3"/> </div>
A call to document.getElementById(‘example’).getElementsByClassName(‘aaa’) would return a NodeList with the two paragraphs p1 and p2 in it.
A call to getElementsByClassName(‘ccc bbb’) would only return one node, however, namely p3.
A call to document.getElementById(‘example’).getElementsByClassName(‘bbb ccc ‘) would return the same thing.
A call to getElementsByClassName(‘aaa,bbb’) would return no nodes; none of the elements above are in the “aaa,bbb” class.
Firefox 3 does support getElementsByClassName. IE8 does not. Could not find if it’s implemented in IE9.