HTML Validator tool
Please see this forum announcement about OnlineWebCheck.com.
Why Validate your HTML?
Valid HTML is usually cross-browser, cross-platform compatible, meaning that it doesn’t matter which browser or device your visitors are using, they will all be able to navigate the site without issues.
Fewer errors in your code means that search engine spiders and crawlers will be able to crawl and index your site faster. While this wouldn’t matter much for smaller sites, but as your website scales and grows, the problem of coding errors compound. With more and more errors stacking up, search engines might find it hard to even crawl, nevermind actually index some pages.