CSS (and arguably HTML and Markdown) only supports multiline comments, and multiline comments are often used for documentation
Those aren't programming languages. Because, especially in HTML, a line end does not end an instruction and is just for formatting. So single line comments would be very awkward in HTML, so they had to use multi only. And because CSS is meant to be used close to HTML, I think they opt in to the same. JavaScript in example has single line comments again.
Does any programming language exist that supports multiline comments only without single line comments?