WebLight is a web site validator that will automatically scan a site, then report on CSS and HTML problems it contains, broken links and more.
Simply point the program at your URL, and it'll crawl through all the pages (support for cookies, custom headers and HTTP authentication means WebLight will reach every corner of the site).
It scans HTML, CSS, RSS and XML files, and more, analyses your code and links, and issues a detailed report on any problems.
You can then filter and categorise the results in a variety of ways, to help you ignore intentionally non-standard code (browser-specific CSS properties, say), and focus on the site's real problems.
And as a bonus, the program can also generate XML sitemaps for your pages, allowing you to set your URL's last modified date, change frequency, priority and more.
WebLight is a straightforward way to track down various web site problems.