The basics are not what make them hard. But they are vast in how they can be used and combined to achieve various results. Writing semantically correct, accessible, maintainable markup and CSS is websites that often have thousands of elements on a page (a separate problem, but true), made up of components authored in a CMS with who-knows-what nesting ... etc etc it’s hard. CSS favors conciseness over explicitness, so there are many properties that look like they do one thing but actually do another. That’s hard too, you have to learn not to assume the obvious interpretation of a property name is the correct one.

What often happens is developers duct tape complicated user interfaces together using the basics (because CSS and HTML are considered “easy”) and end up painting themselves into a corner where it’s increasing difficult to manage the code and assistive technology has no idea what’s going on.

For me, letting new developers know that good HTML and CSS is hard helps them to respect the huge surface area involved and be patient with themselves when they struggle. Telling people those things are “easy” makes them feel stupid when they can’t get something working right away.

I guess you cover this by saying advanced features require some learning - but I feel like people hit the hard part of CSS very early on when trying to implement basic patterns that are on most websites.