window.onload = foc;
function foc() {
   if(document.getElementById("username")){
      document.getElementById("username").focus();
   } 
}

function validate(field) {
var valid = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
var ok = "yes";
var temp;
for (var i=0; i<field.value.length; i++) {
temp = "" + field.value.substring(i, i+1);
if (valid.indexOf(temp) == "-1") ok = "no";
}
if (ok == "no") {
alert("Alphanumeric characters only, please.");
field.focus();
field.select();
   }
}