Because of general incompetence and lack of testing, several sites fail to ensure they compress (either gzip or deflate) the content they send for some browsers but not others; sometimes sites deliberately target browsers for such treatment. This online test checks using a variety of different user-agent strings whether compression browser discrimination occurs. It will follow HTTP redirects, but not meta redirects so that is something to be aware of. Opera users can use this button to easily test a site (drag it up to a toolbar of your choice): GZip-Test
Opera 9.0 : Opera/9.00 (Windows NT 5.1; U; en)
HTTP/1.1 200 OK Date: Thu, 29 Jul 2010 11:15:36 GMT Server: Apache/2.2.3 (CentOS) X-Powered-By: PHP/5.1.6 Content-Encoding: gzip Vary: Accept-Encoding Content-Length: 4657 Content-Type: text/html; charset=iso-8859-1
cURL Time: 1.914381
Opera 9.0 (ID: MSIE 6.0) : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; en) Opera 9.00
HTTP/1.1 200 OK Date: Thu, 29 Jul 2010 11:15:37 GMT Server: Apache/2.2.3 (CentOS) X-Powered-By: PHP/5.1.6 Content-Encoding: gzip Vary: Accept-Encoding Content-Length: 4657 Content-Type: text/html; charset=iso-8859-1
cURL Time: 0.7838
MSIE 6.0 : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
HTTP/1.1 200 OK Date: Thu, 29 Jul 2010 11:15:38 GMT Server: Apache/2.2.3 (CentOS) X-Powered-By: PHP/5.1.6 Content-Encoding: gzip Vary: Accept-Encoding Content-Length: 4657 Content-Type: text/html; charset=iso-8859-1
cURL Time: 0.77216
Firefox 1.0 : Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.8) Gecko/20050511 Firefox/1.0
HTTP/1.1 200 OK Date: Thu, 29 Jul 2010 11:15:39 GMT Server: Apache/2.2.3 (CentOS) X-Powered-By: PHP/5.1.6 Content-Encoding: gzip Vary: Accept-Encoding Content-Length: 4657 Content-Type: text/html; charset=iso-8859-1
cURL Time: 0.792099
Safari v125 : Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/124 (KHTML, like Gecko) Safari/125
HTTP/1.1 200 OK Date: Thu, 29 Jul 2010 11:15:39 GMT Server: Apache/2.2.3 (CentOS) X-Powered-By: PHP/5.1.6 Content-Encoding: gzip Vary: Accept-Encoding Content-Length: 4657 Content-Type: text/html; charset=iso-8859-1
cURL Time: 0.533957