code: function startTagBeforeHead(p, token) {
      const tn = token.tagName;
  
      if (tn === $.HTML) {
          startTagInBody(p, token);
      } else if (tn === $.HEAD) {
          p._insertElement(token, NS.HTML);
          p.headElement = p.openElements.current;
          p.insertionMode = IN_HEAD_MODE;
      } else {
          tokenBeforeHead(p, token);
      }
  }
docs: The "before head" insertion mode ------------------------------------------------------------------