php – 为什么浏览器要求缓存文件？

# BEGIN Compress text files
<ifModule mod_deflate.c>
  <filesMatch ".(css|js|x?html?|php)$">
    SetOutputFilter DEFLATE
  </filesMatch>
</ifModule>
# END Compress text files

# BEGIN Expire headers
<ifModule mod_expires.c>
  ExpiresActive On
  ExpiresDefault "access plus 1 seconds"
  ExpiresByType image/x-icon "access plus 86400 seconds"
  ExpiresByType image/jpeg "access plus 86400 seconds"
  ExpiresByType image/png "access plus 86400 seconds"
  ExpiresByType image/gif "access plus 86400 seconds"
  ExpiresByType application/x-shockwave-flash "access plus 86400 seconds"
  ExpiresByType text/css "access plus 86400 seconds"
  ExpiresByType text/javascript "access plus 86400 seconds"
  ExpiresByType application/javascript "access plus 86400 seconds"
  ExpiresByType application/x-javascript "access plus 86400 seconds"
  ExpiresByType text/html "access plus 600 seconds"
  ExpiresByType application/xhtml+xml "access plus 600 seconds"
</ifModule>
# END Expire headers

# BEGIN Cache-Control Headers
<ifModule mod_headers.c>
  <filesMatch ".(ico|jpe?g|png|gif|swf)$">
    Header set Cache-Control "max-age=86400,public"
  </filesMatch>
  <filesMatch ".(css)$">
    Header set Cache-Control "max-age=86400,public"
  </filesMatch>
  <filesMatch ".(js)$">
    Header set Cache-Control "max-age=86400,private"
  </filesMatch>
  <filesMatch ".(x?html?|php)$">
    Header set Cache-Control "max-age=600,private,must-revalidate"
  </filesMatch>
</ifModule>
# END Cache-Control Headers

# BEGIN Turn ETags Off
<ifModule mod_headers.c>
  Header unset ETag
</ifModule>
FileETag None
# END Turn ETags Off

# BEGIN Remove Last-Modified Header
<ifModule mod_headers.c>
  Header unset Last-Modified
</ifModule>
# END Remove Last-Modified Header

(摘自here)

我正在使用Chrome进行测试,我注意到应该缓存的CSS文件确实没有缓存：

一个随机CSS文件的请求标头是：

Accept:text/css,*/*;q=0.1
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:es-ES,es;q=0.8
Cache-Control:max-age=0
Connection:keep-alive
Cookie:sua_language=en; keep=0; user=0; PHPSESSID=hn5gt5nb1j4sfq1j6m40un3it6; language=es
Host:podo.com
If-Modified-Since:Tue,01 Nov 2011 17:20:59 GMT
Referer:http://podo.com/sheet?id=48
User-Agent:Mozilla/5.0 (X11; Linux i686) AppleWebKit/534.30 (KHTML,like Gecko) Chrome/12.0.742.53 Safari/534.30

并且响应是：

Cache-Control:max-age=86400,public
Connection:Keep-Alive
Date:Tue,28 Feb 2012 17:50:04 GMT
Expires:Wed,29 Feb 2012 17:50:04 GMT
Keep-Alive:timeout=5,max=99
Server:Apache/2.2.15 (Unix) DAV/2 PHP/5.2.10
Vary:Accept-Encoding

我还有以下代码在任何JS文件之前执行：

// Set document header
header('Content-Type: text/javascript; charset=UTF-8');

$expires = ONE_DAY;
header("Pragma: public");
header("Cache-Control: maxage=".$expires);
header('Expires: ' . gmdate('D,d M Y H:i:s',time()+$expires) . ' GMT');

我注意到如果我评论这些行并重新加载页面(而不是通过按F5,使用导航按钮),一些(不是所有)Javascript文件具有与CSS文件相同的行为(被迫重新加载).

发生了什么？为什么CSS文件永远不会被缓存？为什么JS文件有时会被缓存,有时候不是？

更新：我发现如果我评论该行< ifModule mod_expires.c>并且它对应的结束标记,一切都按预期工作,它是,不要求CSS文件(与JS文件一样).现在的问题是：为什么？

更新2：第一次收到CSS文件时的响应头(200响应)：

Accept-Ranges:bytes
Cache-Control:max-age=86400,public
Connection:Keep-Alive
Content-Encoding:gzip
Content-Length:1633
Content-Type:text/css
Date:Thu,01 Mar 2012 16:01:53 GMT
Expires:Fri,02 Mar 2012 16:01:53 GMT
Keep-Alive:timeout=5,max=99
Server:Apache/2.2.15 (Unix) DAV/2 PHP/5.2.10
Vary:Accept-Encoding

请求标头是相同的.