It's always a good idea to test one's code to ensure that it is error-free. Of course, one's tempted to do so by simply looking at it in a browser, but this strategy can be misleading. Test this by looking at the same page using different browsers and computers. Here are a few links that provide more about validation of HTML files.
CAST provides leadership about using technology to improve the lives of people with disabilities.
One of the most important aspects of this effort is making the Web accessible to individuals with disabilities. Bobby checks pages for whether they conform to standards for accessibility. It's a good thing. His system uses perl to examine a page's features and inform the developer about them, so you have to put the perl script on a machine that will run it for you.