For the majority of requests I'd guess the Accept header isn't even used. Do people still write applications that send a PNG if supported, falling back to a GIF otherwise? Or use this to determine the user's language preference? I expect in most requests they are ignored and the server sends whatever it has. APIs would be the only thing I can think of, but even then you could get around that by having different endpoints for different formats. At the cost of saving a few TB (if not more) of Internet traffic a day it seems a fair trade off :-)