I've created a page that will take css (or javascript) files and output them as one file. I have noticed that Firefox and Internet Explorer will both fail to send the "If-Modified-Since" header at any time when I compress the output using ob_gzhandler:
if(!ob_start("ob_gzhandler")) ob_start();
The initial headers (Host and referrer changed in sample):
Response Headers
Cache-Control public, must-revalidate, maxage=4838400
Connection keep-alive
Content-Encoding gzip
Content-Length 87281
Content-Type text/css; charset: UTF-8
Date Wed, 12 Dec 2012 16:04:32 GMT
Expires Wed, 06 Feb 2013 16:04:32 GMT
Last-Modified Fri, 12 Oct 2012 13:47:18 GMT
Pragma public
Server Apache
Vary Accept-Encoding
X-Cache MISS from localhost
X-Powered-By PHP/5.3.13
Request Headers
Accept text/css,*/*;q=0.1
Accept-Encoding gzip, deflate
Accept-Language en-gb,en;q=0.5
Authorization Basic cmVkZnVzZTpyM2RmdXMz
Cache-Control no-cache
Connection keep-alive
Cookie PHPSESSID=e73355c49f06a059c22d7f02687dc51b
DNT 1
Host example.com
Pragma no-cache
Referer http://example.com/
User-Agent Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0
And the following headers are sent on refresh (Host and referrer changed in sample):
Response Headers
Cache-Control public, must-revalidate, maxage=4838400
Connection keep-alive
Content-Encoding gzip
Content-Length 87245
Content-Type text/css; charset: UTF-8
Date Wed, 12 Dec 2012 16:09:11 GMT
Expires Wed, 06 Feb 2013 16:09:11 GMT
Last-Modified Fri, 12 Oct 2012 13:47:18 GMT
Pragma public
Server Apache
Vary Accept-Encoding
X-Cache MISS from localhost
X-Powered-By PHP/5.3.13
Request Headers
Accept text/css,*/*;q=0.1
Accept-Encoding gzip, deflate
Accept-Language en-gb,en;q=0.5
Authorization Basic cmVkZnVzZTpyM2RmdXMz
Cache-Control max-age=0
Connection keep-alive
Cookie PHPSESSID=e73355c49f06a059c22d7f02687dc51b
DNT 1
Host example.com
Referer example.com
User-Agent Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0
If I switch from using ob_gzhandler to a standard ob_start() then it works fine and returns a 304 as expected on the second load.
Another issue that I think is related is that when viewing from an android device, the css doesn't get applied for either the stock browser or for Dolphin Browser, but is fine for Firefox for Android. This is also resolved by disabling the ob_gzhandler
