I give myself one html page. No server side code.
The AJAX isn't hard, nor is the pop-up page, the problem lies in the "no server side code".
Both IE and FireFox don't like to make XMLHttpRequest cross-domain. That is, it doesn't like being hosted on a http://localhost/test.html or even file host file:///test.html, making a request to thottbot.com.
On IE, you get a security question pop'up everytime you try to make such a call. If the user clicks "OK", he'd be let through.
On FireFox, it's automatically denied unless you try to obtain UniversalBrowserRead privilege. Extremely difficult and potentially a security loophole.
The solution seems bleak.
Use AJAX to query the origin server - without making a cross-domain call. But use the server to make the 'translation' - possibly caching as well.