Even though a web page displays correctly in your browser does not mean that the HTML page content is correct. These errors can cause obscure problems in less popular browsers and have the potential of becoming major events when a new version of a browser becomes available.
Site Vigil can regularly check single pages or a whole web site for validation errors. Using the Analyzer utility lets you find out about all the errors and warnings it has spotted.
Here is an example set of error reports generated by the Site Vigil Analyzer :
Tag nesting error, expected </A> before </TD>, Line 32 Column 273 
The required attribute TYPE (Type of style e.g. text/css) for <STYLE> has not been given a value, Line 10 Column 2 
The page has not been given a META Description - this may be used by search engines, Line 55 [4f]