HTTP Compression (Accept-Encoding & Content-Encoding) Notes

Browser Support

Browser	Platform	HTTP	Support	Notes
Netscape 3	All	1.0	Unsupported	–
Netscape 4	All	1.0	Broken	–
Netscape 6 & 7	All	1.1	`Accept-Encoding: gzip, deflate, compress;q=0.9`	Processing compressed content only works when requested. Ignores `Content-Encoding: gzip` from the server.
IE 4 – 6?	All?	1.0 or 1.1	`Accept-Encoding: gzip, deflate` sent when in HTTP/1.1 mode.	Processing compressed content only works when requested. Ignores `Content-Encoding: gzip` from the server.
IE 4 – 8	All?	1.1	gzip and deflate (zip only)	–
Firefox 3+?	All?	1.1?	deflate (zip) and deflate (zlip)	–
Opera 3.5	All?	1.0	Unsupported	–
Opera 4	All?	1.1	`TE: deflate, gzip, x-gzip, identity, trailers`	I.E.: Expected Transfer Encoding instead Content Encoding.
Opera 5+	All?	1.1	`Accept-Encoding: deflate, gzip, x-gzip, identity, *;q=0`	–
Opera 9+?	Win/All?	1.1?	deflate (zip) and deflate (zlip)	–
Browser 1.6	Android	1.1?	`Accept-Encoding: gzip`	–
Browser/Internet ?	Android API Level 9+	1.1?	deflate (zlip only?) and gzip	–
Chrome 3+?	Win/OS X	1.1?	deflate (zip) and deflate (zlip)	–
Safari	iPhone 3.1	1.1?	deflate (zip) and deflate (zlip)	Probably supports gzip as well.
Safari 3+?	Win/OS X	1.1?	deflate (zip) and deflate (zlip)	Probably supports gzip as well.
Lynx 2.6+	All?	gzip only?	–

HTTP Compression Specs

HTTP/1.1 (RFC 2068) – Original v1.1 spec.
HTTP/1.1 (RFC 2616) – Update includes HTTP Compression and says that DEFLATE should reference RFC 1950 (ZLIB) not RFC 1951 (zip)

DEFLATE (RFC 1951) – PKWare Zip compression. Content-Encoding: deflate
DEFLATE (RFC 1950) – ZLIB compression for streaming or communication. Content-Encoding: deflate (not supported by IE8 or lower)
GZIP (RFC 1952) – filename.gz :: Content-Encoding:x-gzip or gzip
COMPRESS – filename.Z :: Content-Encoding:x-compress – UNIX compress() uses LZW compression like the GIF image format.

Tests

http://us.php.net/manual/en/function.gzcompress.php#106239

0: 0.000373 - 82.08 kB (100.02%)	82.08 / 0.000373 = 220,053.62~
1: 0.000914 - 19.61 kB  (23.90%)	19.61 / 0.000914 = 21,455.14~
2: 0.000951 - 18.88 kB  (23.01%)	18.88 / 0.000951 = 19,852.79~
3: 0.000999 - 18.43 kB  (22.46%)	18.43 / 0.000999 = 18,448.45~
4: 0.001498 - 17.65 kB  (21.51%)	17.65 / 0.001498 = 11,782.38~
5: 0.001744 - 17.09 kB  (20.82%)	17.09 / 0.001744 =  9,799.31~
6: 0.002060 - 16.88 kB  (20.57%)	16.88 / 0.002060 =  8,194.17~
7: 0.002233 - 16.85 kB  (20.53%)	16.85 / 0.002233 =  7,545.90~
8: 0.002808 - 16.71 kB  (20.36%)	16.71 / 0.002808 =  5,950.85~
9: 0.002928 - 16.71 kB  (20.36%)	16.71 / 0.002928 =  5,706.97~

References

HTTP compression on Wikipedia
HTTP Compression by Constantin Rack
Which browsers can handle Content-Encoding: gzip ?
Deflate compression browser compatibility and advantages over GZIP
HTTP Compression