autolabel.js

// In most browsers, a <label> tag around a form field, e.g.:
//
//   <label>
//     <input type="checkbox" id="private" name="private">
//     This message is private
//   </label>
//
// works just as if you'd specified a for= attribute on the label
// for the first visible field inside the label.  In Safari 4, this
// seems not to be the case (i.e. the for= attribute is required).
//
// The following code watches the page for all clicks on <label>s 
// and automatically sets the for= attribute, if not present, to 
// the first visible form field.

document.observe("mousedown", function(event) {
  var label = event.findElement("label");
  if (label && !label.hasAttribute("for")) {
    var element = label.down("input[type=radio], input[type=checkbox], input[type=submit]," +
                             "input[type=text],  input[type=password], select, textarea");
    if (element) {
      label.writeAttribute("for", element.identify());
    }
  }
});