Compressing an already-compressed file can make it bigger due to metainfo and the fact the gzip algorithm isn't optimised as well for images as say, jpeg. Furthermore, it's an unnecessary performance overhead on both server and browser (even regular compression is sometimes undesirable for this reason).
I guess the browsers figure it's not worth building in support for a feature that's rarely used and creates a sub-optimal UX.
It's not a matter of optimization. Compression removes redundancy. A compressed file will have very little redundancy, so compressing it again is largely a waste of CPU cycles. This is supported by mathematical principles (cf. information theory).
@2: well, they just gave you some good reasons not to compress files that already are. they want to save resources.
so why don't they unzip compressed files that should not be compressed, though they obviously know how to do it?
because nobody would care if they did it!
You want to know why people don't gzip their images? What benefit do you think you get from gziping a jpeg or png stream? Why were you trying it in the first place?
Take a moment and compare your response to the other responses here. The other two (at the time of this writing) of them are useful to help someone understand something.
The author wrote up his thoughts as a blog post and took the time to submit it looking for help understanding an issue.
You were unnecessarily rude and condescending in your response. I'm only bringing this up because your account is new. If you're going to take the time to create an account to reply to a post, why would you create an account just to leave this comment?
The OP is demonstrably an idiot. He calls himself "Developer, CEO of The Student Cloud", and yet he apparently has no clue whatsoever about the most basic compression algorithms. Zipping compressed images is something that my mother does.
Actually, I think the idiot in the room here is you.
Read his response again. The author is asking why, if Chrome does not support gzipped images, it sends "Accept:/ Accept-Encoding:gzip,deflate,sdch" (in other words it states that it will accept any Gzipped content). This is the question no one in this thread is actually answering, instead choosing to nitpick on why the author tried to do something.
Also, calling someone an idiot because they lack understanding of a certain field in computer science? Really? Maybe you should lighten up.
When the request is sent Chrome doesn't know the content type of the response, so it can't know that what it's requesting is an already-encrypted file.
Good question.
Why doesn't chrome decompress it when their header suggests they will?
Yes it's unneeded and discouraged, but if developers do send gzipped content such as images (which as we all (including the blogger) know, they definitely shouldn't!) surely chrome should just go ahead and decompress it as normal?
EDIT: My question was in reference to the latter part of the final conclusion, Chrome sends:
Accept:/ Accept-Encoding:gzip,deflate,sdch
But doesn't actually support it. Thanks to hobbit_longon for help pointing out the error in the final point.