- * Compliant browsers should not cache this due to the "Cache-Control"
- * setting. However, to be certain, we also set both "Last-Modified"
- * and "Expires" to the current time.
+ * Setting "Cache-Control" to "no-cache" and "Expires" to
+ * the current time doesn't exactly forbid caching, it just
+ * requires the client to revalidate the cached copy.
+ *
+ * If a temporary problem occurres and the user tries again after
+ * getting Privoxy's error message, a compliant browser may set the
+ * If-Modified-Since header with the content of the error page's
+ * Last-Modified header. More often than not, the document on the server
+ * is older than Privoxy's error message, the server would send status code
+ * 304 and the browser would display the outdated error message again and again.
+ *
+ * As a last resort we set "Last-Modified" to Tim Berners-Lee's birthday,
+ * which predates the age of any page on the web and can be safely used to
+ * "revalidate" without getting a status code 304.
+ *
+ * There is no need to let the useless If-Modified-Since header reach the
+ * server, it is therefore stripped by client_if_modified_since in parsers.c.