Intent to prototype: NavigationPreloadManager API

[Perry Jiang]

*Summary: *While ServiceWorkers are intended to improve the performance of
websites, it's possible for them to be counterproductive to performance.
When navigating to a website with a registered ServiceWorker handling fetch
events, the browser must first start up the ServiceWorker before the
ServiceWorker has an opportunity to provide a response. If the
ServiceWorker doesn't provide a response or simply performs a fetch to the
initial target URL, the network response time is unnecessarily delayed by
the ServiceWorker's start up time.

The idea of "navigation preload" addresses this issue; this API enables the
browser to perform a "preload request" for navigations that are potentially
handled by a ServiceWorker before executing the ServiceWorker's fetch
handler. An associated "preload response" will be available to the
ServiceWorker in its fetch handler via a Promise; the ServiceWorker may use
the preload response if it doesn't have a response of its own to provide.
By making ServiceWorker start up and the preload request concurrent, this
API effectively reduces the worst-case network response time as described
above from O(ServiceWorker start up time + network request time) to
O(max(ServiceWorker start up time, network request time)).

Additionally, to allow servers to distinguish between preload requests and
non-preload requests, a "Service-Worker-Navigation-Preload" HTTP header
will be included with each preload request. Users of the API will have
control over this header's value.

*Bug:* https://bugzilla.mozilla.org/show_bug.cgi?id=1290958

*Standard:*
Discussion: https://github.com/w3c/ServiceWorker/issues/920
Interface: https://w3c.github.io/ServiceWorker/#navigation-preload-manager
Integration with "Handle Fetch":
https://w3c.github.io/ServiceWorker/#ref-for-service-worker-registration-navigation-preload-enabled-flag%E2%91%A2

*Platform coverage:* All.

*Preference:* Enabling this feature will require two preferences:
dom.serviceWorkers.navigationPreload.enabled=true (false by default)
dom.serviceWorkers.parent_intercept=true (true by default on Nightly;
targeting true by default on Beta/Release for Firefox 73)

*DevTools bug:* https://bugzilla.mozilla.org/show_bug.cgi?id=1601789

*Other browsers:*
Chrome: shipped since Chrome 59
https://www.chromestatus.com/feature/5734842339688448

Safari: considering/public support
https://bugs.webkit.org/show_bug.cgi?id=182852
https://github.com/w3c/ServiceWorker/issues/920#issuecomment-236251448

*web-platform-tests:*
https://github.com/web-platform-tests/wpt/tree/master/service-workers/service-worker/navigation-preload

*Secure contexts:* Yes.

*Is this feature enabled by default in sandboxed iframes?* Yes, and it does
preserve the invariants in terms of what sandboxed iframes can/can't do.
--
Perry Jiang <pe...@mozilla.com>