code: function (containerEl) {
      // Only a single child element
      if (
        containerEl.childNodes.length === 1
        && containerEl.childNodes[0].nodeType === NODE_TYPE.ELEMENT
      ) {
        return true;
      }
      // Only one element, and all other children are whitespace
      var foundElement = false;
      for (var i = 0, count = containerEl.childNodes.length; i < count; i++) {
        var child = containerEl.childNodes[i];
        if (child.nodeType === NODE_TYPE.ELEMENT) {
          if (foundElement) {
            // Encountered an element after already encountering another one
            // Therefore, more than one element at root level
            return false;
          } else {
            foundElement = true;
          }
        } else if (child.nodeType === NODE_TYPE.TEXT && !isEmpty(child.textContent)) {
          // Contains text content
          return false;
        }
      }
      return true;
    }
docs: Determines if there's only one top-level node in the DOM tree. That is,
  all the HTML is wrapped by a single HTML tag.
  
  @param {DOMElement} containerEl Container element
  @return {boolean}