This evening I did the HTML Developer test provided by W3schools, so now I will get a diploma confirming that I know my html, css and xhtml! http://www.w3schools.com/cert/cert_html.asp
I would highly recommend you to study for it and take the test if you are working as a web developer. Not that the test itself gave me more knowledge, but I learned a lot of new cool stuff when taken the available courses at W3Schools.