W3C HTML Validation Service is a free tool that is invaluable for spotting hidden coding errors. As you can see, the W3C validator instantly caught this problem and flagged it as an error, in red.

This can cause a cascading series of HTML errors and affect how efficiently Google crawls and indexes a web page, particularly with regard to hrfelang tags. Google Says Valid HTML Doesn’t Matter ...

Joydip Kanjilal is a Microsoft Most Valuable Professional (MVP) in ASP.NET, as well as a speaker and the author of several books and articles.