In my opinion, browsers should utilize the .css file's date to determine whether to fetch it from the internet or retrieve it from the cache.
The browser's actions depend on the response headers it receives. For instance, if the Expires header contains a future date, there is no need for the browser to check if the file has been updated. This can be beneficial as it eliminates the need for an additional request to the server.
As a solution for web developers, you can disable the cache in the browser's web tools or regularly clear your cache if this feature is not available in your specific browser.
If you want changes to take effect immediately, one option is to rename the CSS file. Simply adding a modification date parameter to the CSS URL such as
<link rel=stylesheet" href="style.css?20130917" />
, may suffice. However, if you are using a CDN or another layer, you may need to use something like
<link rel=stylesheet" href="style.20130917.css" />
along with some server-side configurations to map filename.[numbers].css to filename.css.