append (children) {
    if (arguments.length === 1) {
      if (isArray(children)) {
        children = children.slice()
      } else {
        const child = children
        return this.appendChild(child)
      }
    } else {
      children = arguments
    }
    if (children) {
      Array.prototype.forEach.call(children, this.appendChild.bind(this))
    }
    return this
  }
Append a child element.

    @param {DOMElement|String} child An element or text to append
    @returns {this}