Is it true "JavaScript files are not reliably cached by browsers, CSS files are"

Question

as mentioned on this site

Note that while JavaScript files are not reliably cached by browsers, CSS files are.

http://www.websiteoptimization.com/speed/tweak/http/

Answer 1

I can't see any reason to make that claim expressly for JavaScript files. I can, however, see an argument made that caching can be unreliable in general regardless of the file type sent, depending on the server configuration, additional headers that are sent, proxies and caches, and how the end-user's browser is configured.

Answer 2

The browsers I know of "reliably cache" all kinds of static data (including JS and CSS, as well as images, HTML, etc) as long as they're served with proper cache-support headers. Maybe the text means something different than actual caching, such as parsing just once and then keeping some efficient internal format...? I don't know which browsers do or don't do that for different kinds of files, but at least under this hypothesis I can see why (e.g) CSS might be easier for the browser to keep in preprocessed form than JS.