I am trying to create user registration form in js which beautiful and flexible
You can do it by using these modules :

Fancy Login


  Fancy Login adds javascript based, lightbox-style login functionality
  to your site. When the user clicks on any link that links to
  /user/login, they screen dims and a login box is created in the center
  of the screen.


Better Login


  Styling Drupal login/password/registration forms are a mission most of
  the time because they form part of the theme... This module removes
  that issue, it is slightly inspired by the way Wordpress login forms
  work, and uses three page templates to change the style of the forms.
  It is very simple to use, just install the Better Login module and it
  starts working immediately.


Model Form


  Modal forms make use of the modal feature in the ctools module to open
  some common forms in a modal window.