This shows two examples of client side validation in a form using JavaScript (with jQuery). The username will check with the server whether the chosen name is a) valid and b) available. The avatar example tries to load the URL in to a hidden image, if it fails, it shows the appropriate message.
Read the article this demonstration relates to
Example of failing username and successful avatar
See the non-JavaScript version
Note that the following usernames are permanently unavailable for the purpose of this demo: remy, julie, andrew, andy, simon, chris, nick