For nav bar hover - consider changing the text color to some light color. Right now, on hover, it’s impossible to read it,
On smaller width, around <600 px, how about either moving the nav bar to the top, or greatly reducing it’s size? Right now the text get’s too narrow and is hard to read as well.
There are HTML coding errors you should be aware of and address.
Since copy/paste from codepen you can ignore the first warning and first two errors. You can also ignore the warnings about the articles lacking a header if you want.
The page needs to be responsive. There’s a horizontal scrollbar on smaller screens.
Remember, the R in RWD stands for Responsive.
A suggestion. The doc is about CSS. In your CSS you have declarations that are multi-line and they’re easy read.
In your doc you have CSS that you display on one line and that’s hard to read.
You can nest multi-line <code> snippets in <pre> </pre> tags in HTML to preserve whitespace and line breaks.
Or skip the <pre> tag and do the following in CSS;