On a server it can be useful to know that an incoming request is AJAX.

Most js libraries use `XMLHttpRequest` and so provide `HTTP_X_REQUESTED_WITH: XMLHttpRequest`, but neither Chrome's implementation nor [Github's polyfill](https://github.com/github/fetch) of the new [`fetch`](https://fetch.spec.whatwg.org) uses a similar header. So how can one detect that the request is AJAX?

Why is a request identifying its initiator not enforced through standards for `fetch` and `XMLHttpRequest`? Should something else be used for decision-making (e.g. clients providing the content-type they expect in response)?