Is "Expires“ in http header useful?

Is there a default behavior for cache of static files like images/js/css in browsers? Even if I did not set the expires on my server, I found that my browser(Firefox, IE8) could also read the static files from local cache, so is the "Expires" use开发者_如何学Pythonful?

Yes, it's absolutely useful. If you don't set the header, the client must guess when the content expires. You may not like how it guesses, and some browsers are more or less aggressive than others. Please read http://blogs.msdn.com/b/ie/archive/2010/07/14/caching-improvements-in-internet-explorer-9.aspx for a full discussion.