The site https://validator.w3.org provides an HTML Validation service that you can (and should) use to validate HTML pages. It's very easy to use. Simply copy your URL from the browser's URL bar and paste it in the address textbox validator, then click "Check."
Missing "lang attribute"
😲 Message: Consider adding a lang attribute to the html start tag to declare the language of this document.
✅ Fix: Include a language attribute in the html start tag:
Character encoding was not declared
😲 Message: The character encoding was not declared. Proceeding using windows-1252.
✅ Fix: Include a meta element with a charset attribute immediately after the opening <head> tag
Start tag seen without doctype
😲 Message: Start tag seen without seeing a doctype first. Expected <!DOCTYPE html>
✅ Fix: Include a !DOCTYPE as the first tag