Inpage SEO Checker is an interesting little library that checks a page against the usual SEO standards.
The library works by testing the page against the usual SEO “good practices” and printing any inconsistencies.
As you can see, the library tests against missing elements (such as title, H1, H2, H3, meta keywords/description) and so on.
All in all, this is a useful tool which I’d like to become a framework; some new features, for example, the chance to check the keyword density would be most welcome.