code: function insertElement(eltFunc) {
      var elt;
      if (foster_parent_mode && isA(stack.top, tablesectionrowSet)) {
        elt = fosterParent(eltFunc);
      }
      else if (stack.top instanceof impl.HTMLTemplateElement) {
        // "If the adjusted insertion location is inside a template element,
        // let it instead be inside the template element's template contents"
        elt = eltFunc(stack.top.content.ownerDocument);
        stack.top.content._appendChild(elt);
      } else {
        elt = eltFunc(stack.top.ownerDocument);
        stack.top._appendChild(elt);
      }
  
      stack.push(elt);
      return elt;
    }
docs: Insert the element into the open element or foster parent it