However, one could use this approach: download as usual, and in a streaming fashion process the data and if it's a progressive JPEG, you can close the connection before you have received everything; and then you can cache the prefix and later download the rest if needed.
Fast clients will just swallow the whole file, while slow clients would be able to benefit from it.
It wouldn't work for pipelined HTTP connections though without cancelling the whole pipeline, so maybe not a very practical solution given the performance benefit that already gives. And HTTP/2 maybe doesn't support cancelling a transfer either, so.. ?
Maybe a direct "Accept" or "Prefer" header to indicate that it's enough to send just something useful for an icon would be a more ideal solution, but it would require server-side support.
However, one could use this approach: download as usual, and in a streaming fashion process the data and if it's a progressive JPEG, you can close the connection before you have received everything; and then you can cache the prefix and later download the rest if needed.
Fast clients will just swallow the whole file, while slow clients would be able to benefit from it.
It wouldn't work for pipelined HTTP connections though without cancelling the whole pipeline, so maybe not a very practical solution given the performance benefit that already gives. And HTTP/2 maybe doesn't support cancelling a transfer either, so.. ?
Maybe a direct "Accept" or "Prefer" header to indicate that it's enough to send just something useful for an icon would be a more ideal solution, but it would require server-side support.