Index: xwiki-core/src/main/java/com/xpn/xwiki/XWiki.java
===================================================================
--- xwiki-core/src/main/java/com/xpn/xwiki/XWiki.java	(revision 9615)
+++ xwiki-core/src/main/java/com/xpn/xwiki/XWiki.java	(working copy)
@@ -906,6 +907,14 @@
         return Util.getFileContent(new InputStreamReader(is));
     }
 
+    public Date getResourceLastModificationDate(String name)
+    {
+        if (getEngineContext() != null) {
+            return Util.getFileLastModifcationDate(getEngineContext().getRealPath(name));
+        }
+        return new Date();
+    }
+
     public byte[] getResourceContentAsBytes(String name) throws IOException
     {
         InputStream is = null;
Index: xwiki-core/src/main/java/com/xpn/xwiki/util/Util.java
===================================================================
--- xwiki-core/src/main/java/com/xpn/xwiki/util/Util.java	(revision 9615)
+++ xwiki-core/src/main/java/com/xpn/xwiki/util/Util.java	(working copy)
@@ -762,4 +762,8 @@
         return result.toString();
     }
 
+    public static Date getFileLastModifcationDate(String path) {
+        File f = new File(path);
+        return (new Date(f.lastModified()));
+    }
 }
Index: xwiki-core/src/main/java/com/xpn/xwiki/web/SkinAction.java
===================================================================
--- xwiki-core/src/main/java/com/xpn/xwiki/web/SkinAction.java	(revision 9615)
+++ xwiki-core/src/main/java/com/xpn/xwiki/web/SkinAction.java	(working copy)
@@ -33,6 +33,7 @@
 
 import java.io.IOException;
 import java.util.Date;
+import java.util.Arrays;
 
 /**
  * <p>
@@ -195,11 +196,21 @@
             data = context.getWiki().getResourceContentAsBytes(path);
             if (data != null && data.length > 0) {
                 String mimetype = context.getEngineContext().getMimeType(filename.toLowerCase());
+                Date modification = null;
                 if (isCssMimeType(mimetype) || isJavascriptMimeType(mimetype)) {
-                    data = context.getWiki().parseContent(new String(data), context).getBytes();
+                    byte[] newdata = context.getWiki().parseContent(new String(data), context).getBytes();
+                    if (Arrays.equals(newdata, data)) {
+                        modification = context.getWiki().getResourceLastModificationDate(path);
+                    }
+                    else {
+                        modification = new Date();
+                        data = newdata;
+                    }
                 }
-
-                setupHeaders(response, mimetype, new Date(), data.length);
+                else {
+                    modification = context.getWiki().getResourceLastModificationDate(path);
+                }
+                setupHeaders(response, mimetype, modification, data.length);
                 try {
                     response.getOutputStream().write(data);
                 } catch (IOException e) {