The CSS files produced from XenForo don't get the correct last modified http header value. XenForo\\CssOutput::displayCss() returns "Last-Modified: Thu, 01 Jan 1970 00:00:00 GMT" despite:\nheader('Last-Modified: ' . gmdate('D, d M Y H:i:s', $this->_styleModifiedDate) . ' GMT')\n\nQuick fix to change it to page generation time:\n\nheader('Last-Modified: ' . gmdate('D, d M Y H:i:s', time()) . ' GMT');\n\nThe Cache-Control header could include the max-age attribute, as the Expires header is already setup to 2020. Adding an ETag (Enable ETag support for CSS Resource) would be great, too.