code: function startTagInTemplate(p, token) {
      const tn = token.tagName;
  
      if (
          tn === $.BASE ||
          tn === $.BASEFONT ||
          tn === $.BGSOUND ||
          tn === $.LINK ||
          tn === $.META ||
          tn === $.NOFRAMES ||
          tn === $.SCRIPT ||
          tn === $.STYLE ||
          tn === $.TEMPLATE ||
          tn === $.TITLE
      ) {
          startTagInHead(p, token);
      } else {
          const newInsertionMode = TEMPLATE_INSERTION_MODE_SWITCH_MAP[tn] || IN_BODY_MODE;
  
          p._popTmplInsertionMode();
          p._pushTmplInsertionMode(newInsertionMode);
          p.insertionMode = newInsertionMode;
          p._processToken(token);
      }
  }
docs: The "in template" insertion mode ------------------------------------------------------------------