code: @Nonnull
    public static String trimUrl (@Nonnull final CharSequence s)
    {
      // Extract from "url(...)"
      final String sTrimmed = _trimBy (s,
                                       CCSSValue.PREFIX_URL_OPEN.length (),
                                       CCSSValue.SUFFIX_URL_CLOSE.length ()).trim ();
      // Remove the trailing quotes (if any)
      final String sUnquoted = extractStringValue (sTrimmed);
      // Unescape all escaped chars
      return unescapeURL (sUnquoted);
    }
docs: Remove the leading "url(" and the trailing ")" from an URL CSS value. No
  check is performed for the existence of a leading "url("! This method
  should only be called from within the parser.
  
  @param s
  The value to remove the string from.
  @return The trimmed value. Never <code>null</code>.