What does this mean and what is WC3?

Here is some information from WC3 for you to read.
I use validation, it's a quick easy way to chase down potential problems.

http://validator.w3.org/about.html

The HTML Validator is to insure that all of the mark-up you use is conforms to the standards proposed by the World Wide Web Consortium. By using valid mark-up you can be sure that the page you created will display in the manner you wrote it.

It gets everybody on the same page, kind of, sort of.