I am looking for an enterprise-grade service or a tool that can be used to scan / fingerprint websites and notify when major XHTML code changes are detected. The tool should be able to continuously scan thousands of websites and determine the percentage of HTML code that has been modified since the last run. And then either save the data where it can be easily accessed or send periodic notifications.

I know of services like ChangeDetect.com, but they don't do markup only changes and instead focus on everything, including content. We don't really care about presentation content, because a lot of sites we need to cover are updated frequently with content.