We need to add one last bit of coding to our web pages. This is the DOCTYPE tag, and tells web browsers what sort of web page you have created. The tag for our pages looks like this:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd ">
What this says to web browsers is that this is a World Wide Web Consortium-okay document that is using HTML 4.01 transitional coding. You can copy and paste this into your index.html, as well as any other pages you may have created. It goes at the very top of your file, above the opening HTML tag. See my index.html  file for an example.
As you write HTML code, you will make mistakes -- everybody does. That is why there are HTML Validators (or checkers). They look at your HTML code and find errors like missing or misplaced tags.
To check your index.html file, you will use W3C HTML Validation Service because it will check the HTML code for files that are on your local computer.
Congratulations! You now have an error free web page!
Checking for Accessibility
Another issue to think about when you build web pages is accessibility. Special needs users may not be able to see pictures on your pages, or may use special web browsers that don't display code the way most web browsers do. You should be designing pages that can be used by everybody.
To read more about current standards and how to make your pages accessible, check out the Usability.gov website . You will find useful links to design guidelines, checklists, and advice on what you can do to your pages to make them more accessible.