code: function doctypeInInitialMode(p, token) {
      p._setDocumentType(token);
  
      const mode = token.forceQuirks ? HTML.DOCUMENT_MODE.QUIRKS : doctype.getDocumentMode(token);
  
      if (!doctype.isConforming(token)) {
          p._err(ERR.nonConformingDoctype);
      }
  
      p.treeAdapter.setDocumentMode(p.document, mode);
  
      p.insertionMode = BEFORE_HTML_MODE;
  }
docs: The "initial" insertion mode ------------------------------------------------------------------