CSS files usually don't change very frequently, and when they do change, then normal server requests should handle it effectively.
That's not always the case. I've seen plenty of times when a new CSS file is available, but the browser continues to serve up a cached one.
The above example is how you could implement a "cache killer". In other words, when you update the CSS file, you also update the HTML and change the value after the ?. This will force the browser to request the file again and see that it has changed. Often times the value is a timestamp, but it really could be any value as long as it can be changed to be a new unique value. For example, you could do: