code: @Nullable
    public static String getURLValue (@Nullable final String sValue)
    {
      if (isURLValue (sValue))
      {
        return CSSParseHelper.trimUrl (sValue);
      }
      return null;
    }
docs: Extract the real URL contained in a CSS URL value.
  
  @param sValue
  The value containing the CSS value
  @return <code>null</code> if the passed value is not an URL value
  @see #isURLValue