diff --git a/serendipity.css.php b/serendipity.css.php
index 4ba23c13..ece63b31 100644
--- a/serendipity.css.php
+++ b/serendipity.css.php
@@ -66,6 +66,10 @@ if (IS_installed === false) {
     die();
 }
 
+// Use output buffering to capture all output. This is necessary
+// because a plugin might call 'echo' directly instead of adding
+// the desired output to the hook parameter '$out'.
+ob_start();
 
 // First all of our fallback classes, so they can be overridden by the usual template.
 $out = serendipity_printStylesheet(
@@ -82,4 +86,35 @@ serendipity_plugin_api::hook_event($css_hook, $out);
 
 echo $out;
 
+//
+// Fetch output buffer containing the CSS output and create eTag header
+//
+$ob_buffer = ob_get_contents();
+
+if ($ob_buffer) {
+
+    // Calculate hash for eTag and get request header. The hash value
+    // changes with every modification to any part of the CSS styles.
+    // This includes the installation of a plugin that adds plugin
+    // specific styles.
+
+    // Send ETag header using the hash value of the CSS code
+    $hashValue = md5($ob_buffer);
+    @header('ETag: "' . $hashValue . '"');
+
+    // Compare value of If-None-Match header (if available) to hash value
+    if (!empty($_SERVER['HTTP_IF_NONE_MATCH'])) {
+        // Get request header value and chop off optional quotes
+        $reqHeader = trim($_SERVER['HTTP_IF_NONE_MATCH'], '"');
+
+        if ($hashValue === $reqHeader) {
+            // Tell client to use the cached version and destroy output buffer
+            @header('HTTP/1.1 304 Not Modified', true, 304);
+            ob_clean();
+        }
+    }
+}
+
+ob_end_flush();
+
 /* vim: set sts=4 ts=4 expandtab : */