As you have pointed out, but formulated more explicitly: client-side rendering should deal only with HTML/CSS because that's what the browser is built and optimized for. Every line of script changing the DOM (html structure) may trigger a redraw of the page, which means wasting considerable amount of resources! But even if your script outputs HTML only once, you still have O(n) HTML templating instead of O(1) for n clients. Such a waste!