[jQuery] (validate) form field validation with field id doesn't work.

Hi :
I am trying to use jquery validatro plugin and used tried demo on
the live site. Just looking if I can use this framework for UI
validation or not.
So, On demo, whene i tried replacing name with id, the validation
doesn't work.
Demo page : has 2 form.
http://jquery.bassistance.de/validate/demo/

And in the second form, when I change the firstname :

<label for="firstname">Firstname</label>
<input id="firstname" name="firstname" />

to something like :
<label for="firstname">Firstname</label>
<input id="firstname" name="firstname1" />

it doesn't work. Please let me know if I need to do anything.