↪ You don't, and you won't, as there is no such a thing as "user registration" on the refererbox site. To use the service, just copy&paste the html into your site pages, and you're done. Don't forget to check the terms of use of the service.

↪ There is just no need for any kind of ID, secret or not: everything is managed internally by the provided scripts. The data itself is public, and does not need any specific protection. However, there is a control based on the IP address of the visitors, that prevents a given address to be counted twice. Although it may provide inaccurate results for visitors using a proxy, it prevents spamming and at the same time enhances the quality of the refering stats by counting only unique visits.

↪ The refererbox widgets never degrade the validity of your html or xhtml, as it uses dynamic DOM injection. Consequently, not only does the html/xhtml totally respect the version of the html used by your page, but the result is also always valid and compliant with your page encoding charset.

↪ Absolutly, and there is nothing for you to do. Once refererbox detects that you stopped using the service for 10 days, all the data related to your pages are automatically deleted. As a side effect, if your site has no visitor for 10 days, your stats are unfortunately erased.

↪ Of course. The refererbox widget are provided under the terms of the Gnu GPL license, and you're free to use, modify, and redistribute them at will (please refer to the license for specific details).