Class 4: Address Form Assignment

My solution for the address assignment, here.
After reading Joel L. post, I decided to add the onchange event handler to the billing address input tags. So, any changes in the billing address inputs will change the shipping address inputs respectively. Also, I added the disabled attribute to the shipping address input tags. The disabled attribute disables the input text box, so the user is unable to make any changes.
The shipping address inputs has a simple validation that would prompt the user if the any of the input fields are left blank.