The World Wide Web Consortium is the body which sets standards for the Web. If they say your code is correct then it is. They offer a validation service for your code. To try it copy this code:
<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>A less than awesome page</title> </head> <body> <p>Hi</p> </p>Problem?</p> </body> </html>
Then click on this link to the official W3C validator. Paste the code in and press the button. It should show two errors but actually there is just one. The second error is caused by the first. The error is described lower down on the validation page and it should tell you to look at line 9 for the problem. Hopefully you can spot it yourself. Fix the problem, copy the new version of the code and paste it into the validator again. Both errors should go away.
Now check your pages (especially index.html, product1.html and address.html). Fix any errors if you can. If you can't fix them it suggests that you are not ready to move on yet so do the practical again (delete all the pages and start again). Or even do the whole HTML tutorial again. There is no point moving on until you understand what you have learned so far.
You might want to bookmark the W3C validation page for later use.
Now you are ready to learn CSS so press the right arrow above. Don't forget to keep practising HTML though.