preetyPhoto jquery is working with html but not with xhtml

Question

$(document).ready(function(){
$("a[rel^='prettyPhoto']").prettyPhoto();
 });

<a href="images/img15.png" rel="prettyPhoto[gallery1]">
     <img style="width: 312px; height: 312px;" src="images/img15.png" alt="Image15"/>
</a>

I have implement code for image popup using preetyPhoto jquery.It is working fine in html but not working in xhtml. Can anyone know why it happens ?