This particular example isn't very convincing. All HTMX needs to send back is a button element (or DIV with button inside it, whatever) with a class attribute.

This particular example isn't very convincing. All HTMX needs to send back is a button element (or DIV with button inside it, whatever) with a class attribute. You already said the styling is based on what the server returns, so the server is perfectly capable of applying the correct class. Let client-side CSS do the styling, the server doesn't need to know anything.