This form uses AJAX to check, while you are filling out the rest of the form, whether the username you have supplied has already been taken. To see it in action, fill out the form as you typically would. For testing purposes, you will get a message either way (so that you know it worked). Note: when you submit the form, it will just kick you back to this page, as this is just a demonstration.

The AJAX related code is added to the document via Javascript and DOM, so there would be no problems if the required JS is not supported in your browser (however, since the form doesn't actually submit to anything, you won't find this page very interesting if your browser doesn't support AJAX related Javascript). Typically, the Javascript would be in an external script file, but it is included inline here for demonstration purposes (same with the CSS). You can easily see how it could be cut and pasted into an external file, because it uses the DOM and is not at all intermingled with the markup.