code: @Nonnull
    public static String getMinifiedCSSFilename (@Nonnull final String sCSSFilename)
    {
      if (!isCSSFilename (sCSSFilename))
        throw new IllegalArgumentException ("Passed file name '" + sCSSFilename + "' is not a CSS file name!");
      if (isMinifiedCSSFilename (sCSSFilename))
        return sCSSFilename;
      return StringHelper.trimEnd (sCSSFilename, CCSS.FILE_EXTENSION_CSS) + CCSS.FILE_EXTENSION_MIN_CSS;
    }
docs: Get the minified CSS filename from the passed filename. If the passed
  filename is already minified, it is returned as is.
  
  @param sCSSFilename
  The filename to get minified. May not be <code>null</code>.
  @return The minified filename