Having valid coding for your site is important. It could mean the difference of a visitor being able to access and view your site properly. Whether you are just learning HTML, know a little bit, or know quite a few tricks, one of the best tools a designer and developer can use is the W3 Markup Validation Service by the World Wide Consortium.
The World Wide Consortium (also known as W3 or W3C) has been around for quite some time and is the governing body to consult about web accessibility and semantic coding. No matter if you are a novice or a seasoned professional in web design, the validator is a great tool for both learning and also correcting those minor mistakes missed the first time.
From my experience, many people who have used it have asked me how can they tell where to correct a mistake.
Below is a short video run through to help you through understanding how to use the W3C Markup Validation Service.