If you want to explain how markup is different from a general purpose language, you can always refer to the fact that markup isn't turing complete, and can't be used to perform arbitrary loops and arithmetic.

But that would also make things like basic SQL 'not coding', and any program you wrote where you didn't use an arbitrarily long loop.

That makes "print('Hello world')" not coding, and then it becomes clear that you're drawing an arbitrary line in the sand. "If you aren't writing compilers, you're not coding" or some such nonsense.

HTML and CSS are languages for encoding the semantics of webpage formatting and content. They are domain specific languages. They are not turing complete. But they are coding.