function _getDefaultImpl () {
  if (DefaultDOMElement._forceMemoryDOM) {
    return MemoryDOMElement
  } else if (platform.inBrowser || platform.inElectron) {
    return BrowserDOMElement
  } else {
    return MemoryDOMElement
  }
}
A Bridge to the default DOMElement implementation, either BrowserDOMElement, or MemoryDOMElement.