Not sure that's a very good example to be honest. Why would you need to create an entirely new CSS file for this? And adding a single comment line in addition

Not sure that's a very good example to be honest. Why would you need to create an entirely new CSS file for this? And adding a single comment line in addition to your style sheet isn't exactly going to break the bank. What if your client comes back to you in a few weeks/months and says "I want another playback progress bar on another page"?