En explorer hay que definir document.getElementsByClassName (no es nativa). Probá así:
Código:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script>
if(typeof(document.getElementsByClassName) != 'function') {
document.getElementsByClassName = function (cn) {
var rx = new RegExp("\\b" + cn + "\\b"), allT = document.getElementsByTagName("*"), allCN = [], i = 0, a;
while (a = allT[i++]) {
if (a.className && a.className.indexOf(cn) + 1) {
if(a.className===cn){ allCN[allCN.length] = a; continue; }
rx.test(a.className) ? (allCN[allCN.length] = a) : 0;
}
}
return allCN;
}
}
onload=function(){
var c=document.getElementsByClassName('pp'),i=-1;
while(c[++i])
alert(c[i].innerHTML);
}
</script>
</head>
<body>
<span class="pp">a</span>
<span class="pp">b</span>
<span class="pp">c</span>
</body>
</html>