Without knowing good X/HTML we can't use css properly optimize way.....
How to give some example and argument in favor of this... to web-design student Who are trying to learn css more but doesn't know how to use all HTML tag wise-full to work with CSS better.
I am not sure if i understand correctly but one classic resource to look at is CSSzengarden (http://www.csszengarden.com/) i am not sure if it is updated anymore but it is an example of one page styled with just css to make it look completely different in many ways.
It is what inspired me to become competent at CSS styling.
Sounds like you're trying to win an argument with someone. If your position is that said student should learn html in order to use css effectively, I agree with you. But the real answer to your question is "DUH!" (American idiom meaning "there should be no argument").
For the sake of said student, I'll use a construction analogy. HTML is the foundation, walls and roof of any website, regardless of rendering engine (ASP.NET, PHP, Ruby, Python, etc.). CSS is the paint on the walls (among other useful things). If you build a building with crappy foundation, walls, and roof, you have a crappy building no matter how cool the paint looks. Websites are the same way and said student should learn how to build the entire website and take pride in his/her work instead of trying to take shortcuts.
I feel better now.