The World Wide Web Consortium (W3C) is an international community where Member organizations, a full-time staff, and the public work together to develop Web standards.
W3C HTML Validator is a SEO tool to validate the HTML code of your website as per standard maintained by W3C.
Since all website browsers follow HTML standard so if your website is not properly structured using HTML standards it will not be properly load in the browsers and later once google or other search engines crawl they will feel difficult to crawl your website and do proper indexing.
To validate your website click the below link
This will open below page
Enter your website URL and click check.
Once the test completes it will show you result where you will find set of errors and warnings.[/pl_text] [/pl_col] [/pl_row]