A lot of caching happens in the web browser, which is inconvenient because you don't have direct control over it (unless it's your browser). Asking users to clear their cache is unsatisfactory and confusing to many. Yet, you can exert influence on how browsers cache resources by carefully controlling the HTTP headers that you set and the URLs that you use.
If you fail to declare what resources are cacheable and for how long, then many browsers will just guess this. The heuristics for this can be wildly different between implementations. Therefore, this will result in sub-optimal performance. You should be explicit and always declare cache information even (and especially) for assets that shouldn't be cached by marking them as non-cacheable. ...
