Browser Caching of CSS files

To the first part of your question - yes, browsers cache css files (if this is not disabled by browser's configuration). Many browsers have key combination to reload a page without a cache. If you made changes to css and want users to see them immediately instead of waiting next time when browser reloads the files without caching, you can change the way CSS ir served by adding some parameters to the url like this:

/style.css?modified=20012009

It does depend on the HTTP headers sent with the CSS files as both of the previous answers state - as long as you don't append any cachebusting stuff to the href. e.g.

<link href="/stylesheets/mycss.css?some_var_to_bust_cache=24312345" rel="stylesheet" type="text/css" />

Some frameworks (e.g. rails) put these in by default.

However If you get something like firebug or fiddler, you can see exactly what your browser is downloading on each request - which is expecially useful for finding out what your browser is doing, as opposed to just what it should be doing.

All browsers should respect the cache headers in the same way, unless configured to ignore them (but there are bound to be exceptions)

Your file will probably be cached - but it depends...

Different browsers have slightly different behaviors - most noticeably when dealing with ambiguous/limited caching headers emanating from the server. If you send a clear signal, the browsers obey, virtually all of the time.

The greatest variance by far, is in the default caching configuration of different web servers and application servers.

Some (e.g. Apache) are likely to serve known static file types with HTTP headers encouraging the browser to cache them, while other servers may send no-cache commands with every response - regardless of filetype.

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So, first off, read some of the excellent HTTP caching tutorials out there. HTTP Caching & Cache-Busting for Content Publishers was a real eye opener for me :-)

Next install and fiddle around with Firebug and the Live HTTP Headers add-on , to find out which headers your server is actually sending.

Then read your web server docs to find out how to tweak them to perfection (or talk your sysadmin into doing it for you).

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As to what happens when the browser is restarted, it depends on the browser and the user configuration.

As a rule of thumb, expect the browser to be more likely to check in with the server after each restart, to see if anything has changed (see If-Last-Modified and If-None-Match).

If you configure your server correctly, it should be able to return a super-short 304 Not Modified (costing very little bandwidth) and after that the browser will use the cache as normal.