

Both HTML and CSS (when together) are turing complete, and JSX syntax only supports HTML comments in many places.
JavaScript having single line comments isn’t really relevant. Multiline comments are in heavy use there, arguably moreso than single line comments.
Edit: and I should add that “programming languages” is moving the goalpost. The person I responded to didn’t qualify their statement to programming languages.




It means neither of those, actually, just that CSS supports only multiline comments.
Multiline comments remain prevalent across ecosystems though (like in JS and Python). If you believe they’re bad there too, that’s fine, but the fact is that it would be an anti-pattern to go against those established standards.