Writing Valid HTML5 Documents
When you write HTML5 or XHTML documents, it’s possible that you make mistakes. Especially when you just get started in web design. You can easily forget to close a tag or use a deprecated tag or attribute you found in an online tutorial. It’s usually very easy to fix these errors. The easiest way to find what errors you made is by using an HTML validator.
Using those tools is very straightforward: point the validator to your online HTML5 document, upload your file, or paste your HTML5 code in the textarea on the website. The validator will tell if your document is valid and if you made any mistakes, it will also tell you the line number and how to fix them.
Note that these HTML5 validators are still experimental and the validation error messages are not as easy to read as the messages of the XHTML validators.