Attempting to access selectionStart of a (radio?) input object gives this:
Error: uncaught exception: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMNSHTMLInputElement.selectionStart]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: <unknown filename> :: onload :: line 0" data: no]
Here's the demo code:
-------------------------
<html>
<head/>
<body onload="xx = document.getElementById ('x'); alert (typeof (xx.selectionStart));">
<form>
<input id="x" type="radio"/>
</form>
</body>
</html>
-------------------------
FF version : 1.0.7
OS: Win98