function getLabelForId(id) {
  var label, labels = document.getElementsByTagName('label');
  for (var i = 0; (label = labels[i]); i++) {
    if (label.htmlFor == id) {
      return label;
    }
  }
  return false;
}
function checkName() {
  var fn = document.getElementById('first_name');
  var ln = document.getElementById('last_name');
  var label = getLabelForId('first_name');
  if ((fn.value.length > 0) && (ln.value.length > 0)) {
    label.className = 'completed';
  } else {
    label.className = 'required';
  }
} 
 function checkEmail() {
  var e = document.getElementById('email_address');
  var label = getLabelForId('email_address');
  if (/\w+@\w+(\.\w+)+/.test(e.value)) {
    label.className = 'completed';
  } else {
    label.className = 'required';
  }
}