Looking at the list of HTTP headers at Wikipedia, you'll notice a few headers like If-Modified-Since, that serve to send content over only if it has changed since the last request.
While working with JavaScrip frameworks like jQuery, the thought occurred to me, that maybe half of all the websites I open are using the exact same JS framework, isn't that a waste of bandwidth?
The best real-life solution would be to get your JS from Google, so you at least share a cached version with others that do the same: http://code.google.com/apis/ajaxlibs/
My imaginary solution however, would allow the same file, served from different hosts to be cached only once.
Look again at the list of headers... We're looking for something to identify files as equal... What about Content-MD5? It exists, right there! But as far as I can tell it's used for content-verification only.
So I want to propose a new HTTP header: If-MD5-Differs
What do you think about it? Cool? Useless? Security issues?