Read the Markup Validator's documentation on icons usage for more information on policies around these icons.
Client side validation is performed by a web browser, before input is sent to a web server.
Then try revalidating your HTML to show what errors are left.
Sometimes fixing an earlier error will also get rid of other error messages — several errors can often be caused by a single problem, in a domino effect.
Writing HTML is fine, but what if something goes wrong, and you can't work out where the error in the code is?
This article will introduce you to some tools that can help you find and fix errors in HTML.