I know this question isn't about c++, but would you mind telling me if knowing HTML5 is necessary for knowing CSS?

Answer 1

HTML5 is not necessary to learn Cascading Style Sheets, however, CSS is mostly used in combination with HTML and having an understanding of HTML would be handy.

Answer 2

You can learn CSS3 without knowing HTML5 but doing so you won't have anything to do with your knowledge. CSS is designed to work with HTML.