Codepen provides the boilerplate for you. It only expects the code you’d put within the body element in HTML. (No need to include the body tags). For anything you want to add to the <head> element click on the ‘Settings’ button, then HTML and add it into the ‘Stuff for <head>’ box.
Mentioning because you have elements placed incorrectly. Everything to browser renders belongs within the body element. That means header and footer. Also, to declare an HTML5 document it’s !DOCTYPE
Codepen provides validators for HTML, CSS and JS. Click on the down arrow in the upper right of each section and then click on the respective ‘Analyze’ link.
The one for HTML misses things which is why I recommend W3C
The one for CSS is good. Use it, there are things to clean up.
Make your page responsive. The video doesn’t scale which results in a horizontal scrollbar on smaller screens.
survey
Same as above re: using codepen properly and having elements placed incorrectly