Without knowing the actual lesson (a link will help us, here) I cannot say whether or not the above code was asked for in the instructions. It supercedes the rule above it, the one that matters. I would remove all of it and keep only the background-size property. It covers the view port, and is centered and fixed in that sense, and does not repeat.
Since you already have a
body selector, this rule can be written in that selector, No need to repeat selectors.
There is a missing
<title></title> element, which is usually the first or second line in the HEAD section.
Here is a typical valid HTML5 document: