In my application the user has the option to modify the CSS of their site.

This is unlikely to change that often, but when it does we need to ensure that they, and their site visitors, see the results instantly.

We do record the date and time that the user updated their CSS so an easy solution would be to just append a timestamp to the url.

However, I would like to know if I can set the cache headers programatically to force the browser to re-request the CSS file if it changes.