function handleExistingNodes(target: HTMLElement) {
  const existingNodes = target.querySelectorAll('template');
  for (let i = 0; i < existingNodes.length; i++) {
    handleNode(existingNodes[i]);
  }
}
This file is compiled to a standalone browser script by rollup and loaded by Fizz
 clients. Therefore, it should be fast and not have many external dependencies.
@flow