text: let opener = window.opener || (window.parent || window);
  // tree builder originally conceived as singleton ( as a simple function ).
  // Later changed as constructor, however at the moment only one instance for document is supported.
  // these below are the variables that are setted in constructor and shared among the functions
  let htmlTreeBuilderTarget; // DOM target
  let showEmptyNodes = null;
  const voidElementsRegex = /^(AREA|BASE|BR|COL|COMMAND|EMBED|HR|IMG|INPUT|KEYGEN|LINK|META|PARAM|SOURCE|TRACK|WBR)$/;
  