Post author here. I agree 100% that HTML and CSS are not programming languages. However, any application that you build on the front-end is going to require your JavaScript code to interact with the HTML and CSS, so a solid understanding of how all the pieces fit together is crucial.

I can't tell you how many times I've seen back-end engineers hack together a 100+ line JavaScript solution to a problem when one line of CSS would have fixed it.

If you don't know CSS, HTML and DOM very well, then you're probably going to be over-engineering your front-end apps.