Problems with ob_gzhandler and CSS files

[snorlaxle]

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 \[code\]ob_gzhandler\[/code\]:\[code\]if(!ob_start("ob_gzhandler")) ob_start();\[/code\]The initial headers (Host and referrer changed in sample):\[code\]Response HeadersCache-Control public, must-revalidate, maxage=4838400Connection keep-aliveContent-Encoding gzipContent-Length 87281Content-Type text/css; charset: UTF-8Date Wed, 12 Dec 2012 16:04:32 GMTExpires Wed, 06 Feb 2013 16:04:32 GMTLast-Modified Fri, 12 Oct 2012 13:47:18 GMTPragma publicServer ApacheVary Accept-EncodingX-Cache MISS from localhostX-Powered-By PHP/5.3.13Request HeadersAccept text/css,*/*;q=0.1Accept-Encoding gzip, deflateAccept-Language en-gb,en;q=0.5Authorization Basic cmVkZnVzZTpyM2RmdXMzCache-Control no-cacheConnection keep-aliveCookie PHPSESSID=e73355c49f06a059c22d7f02687dc51bDNT 1Host example.comPragma no-cacheReferer http://example.com/User-Agent Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0\[/code\]And the following headers are sent on refresh (Host and referrer changed in sample):\[code\]Response HeadersCache-Control public, must-revalidate, maxage=4838400Connection keep-aliveContent-Encoding gzipContent-Length 87245Content-Type text/css; charset: UTF-8Date Wed, 12 Dec 2012 16:09:11 GMTExpires Wed, 06 Feb 2013 16:09:11 GMTLast-Modified Fri, 12 Oct 2012 13:47:18 GMTPragma publicServer ApacheVary Accept-EncodingX-Cache MISS from localhostX-Powered-By PHP/5.3.13Request HeadersAccept text/css,*/*;q=0.1Accept-Encoding gzip, deflateAccept-Language en-gb,en;q=0.5Authorization Basic cmVkZnVzZTpyM2RmdXMzCache-Control max-age=0Connection keep-aliveCookie PHPSESSID=e73355c49f06a059c22d7f02687dc51bDNT 1Host example.comReferer example.comUser-Agent Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0\[/code\]If I switch from using \[code\]ob_gzhandler\[/code\] to a standard \[code\]ob_start()\[/code\] 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 \[code\]ob_gzhandler\[/code\]