code: @Nonnull
    public static String getAsHTMLID (@Nullable final String sSrc)
    {
      String ret = StringHelper.getNotNull (sSrc, "").trim ();
      ret = StringHelper.removeMultiple (ret, ID_REMOVE_CHARS);
      return ret.isEmpty () ? "_" : ret;
    }
docs: Convert the passed ID string to a valid HTML ID.
  
  @param sSrc
  The source string. May be <code>null</code>.
  @return An underscore instead of an empty string. The string cleaned from the
  malicious characters.