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<pre class="metadata">
Title: Prerendering Revamped
Shortname: prerendering-revamped
Group: WICG
Status: CG-DRAFT
Repository: WICG/nav-speculation
URL: https://wicg.github.io/nav-speculation/prerendering.html
Level: 1
Editor: Domenic Denicola, Google https://www.google.com/, [email protected]
Editor: Dominic Farolino, Google https://www.google.com/, [email protected]
Abstract: This document contains a collection of specification patches for well-specified prerendering.
Markup Shorthands: css no, markdown yes
Assume Explicit For: yes
Complain About: accidental-2119 yes, missing-example-ids yes
Indent: 2
Boilerplate: omit conformance
</pre>
<pre class="link-defaults">
spec:html; type:element; text:link
spec:html; type:element; text:script
</pre>
<pre class="anchors">
spec: ecma262; urlPrefix: https://tc39.es/ecma262/
type: dfn
text: current Realm; url: current-realm
spec: html; urlPrefix: https://html.spec.whatwg.org/multipage/
type: dfn
urlPrefix: browsers.html
text: creating a new top-level browsing context; url: creating-a-new-top-level-browsing-context
for: browsing context
text: remove; url: bcg-remove;
text: fully active; url: fully-active
text: top level browsing context; url:
urlPrefix: history.html
text: session history; url: session-history
urlPrefix: browsing-the-web.html
for: session history entry
text: document; url: she-document
for: navigate
text: mature; url: concept-navigate-mature
for: navigation params
text: request; url: navigation-params-request
text: reserved environment; url: navigation-params-reserved-environment
text: abort; for: Document; url: abort-a-document
text: create and initialize a Document object; url: initialise-the-document-object
text: history handling behavior; url: history-handling-behavior
text: prompt to unload; url: prompt-to-unload-a-document
text: refused to allow the document to be unloaded; url: refused-to-allow-the-document-to-be-unloaded
text: traverse the history; url: traverse-the-history
urlPrefix: interaction.html
text: system focus; url: tlbc-system-focus
urlPrefix: urls-and-fetching.html
text: parse a URL; url: parse-a-url
text: resulting URL record
urlPrefix: web-messaging.html
text: BroadcastChannel; url: broadcastchannel
urlPrefix: webappapis.html
text: script; url: concept-script
urlPrefix: workers.html
text: The worker's lifetime; url: the-worker's-lifetime
text: active needed worker; url: active-needed-worker
urlPrefix: media.html
text: playing the media resource; url: playing-the-media-resource
text: current playback position; url: current-playback-position
spec: geolocation; urlPrefix: https://w3c.github.io/geolocation-api/
type: method; for: Geolocation
text: getCurrentPosition(successCallback, errorCallback, options); url: getcurrentposition-method
text: watchPosition(successCallback, errorCallback, options); url: watchposition-method
text: clearWatch(watchId); url: clearwatch-method
type: dfn
text: watch process; url: dfn-watch-process
spec: serial; urlPrefix: https://wicg.github.io/serial/
type: method; for: Serial
text: getPorts(); url: dom-serial-getports
text: requestPort(options); url: dom-serial-requestPort
spec: presentation-api; urlPrefix: https://w3c.github.io/presentation-api/
type: method; for: PresentationRequest
text: start(); url: dom-presentationrequest-start
spec: webmidi; urlPrefix: https://webaudio.github.io/web-midi-api/
type: method; for: Navigator
text: requestMIDIAccess(); url: dom-navigator-requestmidiaccess
spec: file-system-access; urlPrefix: https://wicg.github.io/file-system-access/
type: method; for: Window
text: showOpenFilePicker(); url: dom-window-showopenfilepicker
text: showSaveFilePicker(); url: dom-window-showsavefilepicker
text: showDirectoryPicker(); url: dom-window-showdirectorypicker
spec: idle-detection; urlPrefix: https://wicg.github.io/idle-detection/
type: method; for: IdleDetector
text: requestPermission(); url: api-idledetector-requestpermission
text: start(); url: api-idledetector-start
spec: clipboard-apis; urlPrefix: https://w3c.github.io/clipboard-apis/
type: method; for: Clipboard
text: read(); url: dom-clipboard-read
text: readText(); url: dom-clipboard-readtext
text: write(); url: dom-clipboard-write
text: writeText(); url: dom-clipboard-writetext
spec: screen-wake-lock; urlPrefix: https://w3c.github.io/screen-wake-lock/
type: method; for: WakeLock
text: request(); url: the-request-method
spec: web-nfc; urlPrefix: https://w3c.github.io/web-nfc/
type: method; for: NDEFReader
text: write(); url: dom-ndefreader-write
text: scan(); url: dom-ndefreader-scan
spec: battery; urlPrefix: https://w3c.github.io/battery/
type: method; for: Navigator
text: getBattery(); url: dom-navigator-getbattery
type: dfn
text: battery promise; url: dfn-battery-promise
spec: pointerlock; urlPrefix: https://w3c.github.io/pointerlock/
type: method; for: Element
text: requestPointerLock(options); url: #dfn-requestpointerlock-pointerlockoptions-options
spec: screen-orientation; urlPrefix: https://w3c.github.io/screen-orientation/
type: dfn
text: apply an orientation lock; url: dfn-apply-an-orientation-lock
text: pre-lock conditions; url: dfn-pre-lock-conditions
type: method; for: ScreenOrientation
text: lock(); url: dom-screenorientation-lock
text: unlock(); url: dom-screenorientation-unlock
spec: gamepad; urlPrefix: https://w3c.github.io/gamepad/
type: method; for: Navigator
text: getGamepads(); url: dom-navigator-getgamepads
type: event; for: Window
text: gamepadconnected; url: dfn-gamepadconnected
text: gamepaddisconnected; url: dfn-gamepaddisconnected
type: interface
text: Gamepad; url: dom-gamepad
text: GamepadEvent; url: dom-gamepadevent
type: attribute; for: GamepadEvent
text: gamepad; url: dom-gamepadevent-gamepad
spec: picture-in-picture; urlPrefix: https://w3c.github.io/picture-in-picture/
type: dfn
text: request Picture-in-Picture; url: request-picture-in-picture-algorithm
type: attribute; for: HTMLVideoElement
text: autoPictureInPicture; url: dom-htmlvideoelement-requestpictureinpicture
type: method; for: HTMLVideoElement
text: requestPictureInPicture(); url: dom-htmlvideoelement-autopictureinpicture
spec: navigation-timing; urlPrefix: https://w3c.github.io/navigation-timing/
type: dfn
text: current document; url: dfn-current-document
spec: encrypted-media; urlPrefix: https://w3c.github.io/encrypted-media/
type: method; for: Navigator
text: requestMediaKeySystemAccess(keySystem, supportedConfigurations); url: requestMediaKeySystemAccess
spec: mediacapture-main; urlPrefix: https://w3c.github.io/mediacapture-main/
type: dfn
text: device change notification steps; url: dfn-device-change-notification-steps
spec: hid; urlPrefix: https://wicg.github.io/webhid/
type: interface
text: HID; url: hid-interface
type: method; for: HID
text: getDevices(); url: dom-hid-getdevices
type: method; for: HID
text: requestDevice(); url: dom-hid-requestdevice
spec: backgroud-fetch; urlPrefix: https://wicg.github.io/background-fetch/
type: interface
text: BackgroundFetchManager; url: backgroundfetchmanager
type: method; for: BackgroundFetchManager
text: fetch(); url: dom-backgroundfetchmanager-fetch
spec: push-api; urlPrefix: https://w3c.github.io/push-api/
type: interface
text: PushManager; url: dom-pushmanager
type: method; for: PushManager
text: subscribe(); url: dom-pushmanager-subscribe
spec: screen-capture; urlPrefix: https://w3c.github.io/mediacapture-screen-share
type: method; for: MediaDevices
text: getDisplayMedia(); url: dom-mediadevices-getdisplaymedia
spec: audio-output; urlPrefix: https://w3c.github.io/mediacapture-output/
type: method; for: MediaDevices
text: selectAudioOutput(); url: dom-mediadevices-selectaudiooutput
spec: speech-api; urlPrefix: https://wicg.github.io/speech-api/
type: interface
text: SpeechSynthesis; url: speechsynthesis
type: method; for: SpeechSynthesis
text: speak(utterance); url: dom-speechsynthesis-speak
text: cancel(); url: dom-speechsynthesis-cancel
text: pause(); url: dom-speechsynthesis-pause
text: resume(); url: dom-speechsynthesis-resume
text: getVoices(); url: dom-speechsynthesis-getvoices
type: interface
text: SpeechRecognition; url: speechrecognition
type: method; for: SpeechRecognition
text: start(); url: dom-speechrecognition-start
text: stop(); url: dom-speechrecognition-stop
text: abort(); url: dom-speechrecognition-abort
spec: web-locks; urlPrefix: https://w3c.github.io/web-locks/
type: method; for: LockManager
text: request(); url:dom-lockmanager-request
text: query(); url:dom-lockmanager-query
spec: service-workers; urlPrefix: https://w3c.github.io/ServiceWorker/
type: interface
text: ServiceWorkerRegistration; url: serviceworkerregistration-interface
type: method; for: ServiceWorkerRegistration
text: update(); url: dom-serviceworkerregistration-update
text: unregister(); url: dom-serviceworkerregistration-unregister
type: interface
text: ServiceWorkerContainer; url: serviceworkercontainer-interface
type: method; for: ServiceWorkerContainer
text: register(scriptURL, options); url: dom-serviceworkercontainer-register
type: interface
text: ServiceWorker; url: serviceworker-interface
type: method: for: ServiceWorker
text: postMessage(message, transfer); url: dom-serviceworker-postmessage
text: postMessage(message, options); url: service-worker-postmessage-options
spec: nav-speculation; urlPrefix: prefetch.html
type: dfn
text: supports prefetch; url: supports-prefetch
text: list of sufficiently-strict speculative navigation referrer policies
spec: RFC8941; urlPrefix: https://www.rfc-editor.org/rfc/rfc8941.html
type: dfn
text: structured header; url: #section-1
for: structured header
text: list; url: name-lists
text: token; url: name-tokens
spec: client-hints-infrastructure; urlPrefix: https://wicg.github.io/client-hints-infrastructure
type: dfn
text: Accept-CH cache; url: accept-ch-cache
text: add a new Accept-CH cache entry; url: add-a-new-accept-ch-cache-entry
text: append client hints to request; url: abstract-opdef-append-client-hints-to-request
text: client hints token; url: client-hints-token-definition
text: client hints set; url: client-hints-set
text: update the Client Hints set; url: abstract-opdef-update-the-client-hints-set
for: environment settings object
text: client hints set; url: environment-settings-object-client-hints-set
</pre>
<pre class="biblio">
{
"IDLE-DETECTION": {
"authors": [
"Reilly Grant"
],
"href": "https://wicg.github.io/idle-detection/",
"title": "Idle Detection API",
"status": "CG-DRAFT",
"publisher": "W3C"
}
}
</pre>
<style>
/* domintro and XXX from https://resources.whatwg.org/standard.css */
.domintro {
position: relative;
color: green;
background: #DDFFDD;
margin: 2.5em 0 2em 0;
padding: 1.5em 1em 0.5em 2em;
}
.domintro dt, .domintro dt * {
color: black;
font-size: inherit;
}
.domintro dd {
margin: 0.5em 0 1em 2em; padding: 0;
}
.domintro dd p {
margin: 0.5em 0;
}
.domintro::before {
content: 'For web developers (non-normative)';
background: green;
color: white;
padding: 0.15em 0.25em;
font-style: normal;
position: absolute;
top: -0.8em;
left: -0.8em;
}
.XXX {
color: #D50606;
background: white;
border: solid #D50606;
}
</style>
<h2 id="prerendering-bcs">Prerendering browsing context infrastructure</h2>
<h3 id="document-prerendering">Extensions to the {{Document}} interface</h3>
We'd modify [[HTML]]'s centralized definition of {{Document}} as follows:
<pre class="idl">
partial interface Document {
readonly attribute boolean prerendering;
// Under "special event handler IDL attributes that only apply to Document objects"
attribute EventHandler onprerenderingchange;
};
</pre>
The <dfn attribute for="Document">onprerenderingchange</dfn> attribute is an [=event handler IDL attribute=] corresponding to the <dfn event for="Document">prerenderingchange</dfn> [=event handler event type=]. (We would update the corresponding table in [[HTML]], which currently only contains {{Document/onreadystatechange}}.)
As is customary for [[HTML]], the definition of {{Document/prerendering}} would be located in another section of the spec; we'd place it in the new section introduced below:
<h3 id="prerendering-bcs-subsection">Prerendering browsing contexts</h3>
<em>The following section would be added as a new sub-section of [[HTML]]'s <a href="https://html.spec.whatwg.org/multipage/browsers.html#windows">Browsing contexts</a> section.</em>
Every [=browsing context=] has a <dfn for="browsing context">loading mode</dfn>, which is one of the following:
: "`default`"
:: No special considerations are applied to content loaded in this browsing context
: "`prerender`"
:: This browsing context is displaying prerendered content
By default, a [=browsing context=]'s [=browsing context/loading mode=] is "`default`". A browsing context whose [=browsing context/loading mode=] is "`prerender`" is known as a <dfn>prerendering browsing context</dfn>.
<p class="note">Although there are only two values for the [=browsing context/loading mode=], we use a flexible structure in anticipation of other future loading modes, such as those provided by fenced frames, portals, and uncredentialed (cross-site) prerendering. It's not yet clear whether that anticipation is correct; if, as those features gain full specifications, it turns out not to be, we will instead convert this into a boolean.
<dl class="domintro">
<dt><code>document . {{Document/prerendering}}</code></dt>
<dd>Returns true if the page is being presented in a non-interactive "prerendering-like" context. In the future, this would include a visible document in a `<portal>` element, both when loaded into it or via predecessor adoption.
</dl>
The <dfn attribute for="Document">prerendering</dfn> getter steps are to return true if [=this=] has a non-null [=Document/browsing context=] that is a [=prerendering browsing context=]; otherwise, false.
<hr>
A [=prerendering browsing context=] is <dfn for="prerendering browsing context">empty</dfn> if the only entry in its [=session history=] is the initial `about:blank` {{Document}}.
Every {{Document}} has a <dfn for="Document">prerendering browsing contexts map</dfn>, which is an [=ordered map=] of [=URLs=] to [=prerendering browsing contexts=]. This is used to fulfill [=navigate|navigations=] to a given URL by instead [=prerendering browsing context/activating=] the corresponding prerendering browsing context.
Every {{Document}} has a <dfn for="Document">post-prerendering activation steps list</dfn>, which is a [=list=] where each [=list/item=] is a series of algorithm steps. For convenience, we define the <dfn for="platform object">post-prerendering activation steps list</dfn> for any platform object |platformObject| as:
<dl class="switch">
: If |platformObject| is a [=node=]
:: 1. Return |platformObject|'s [=Node/node document=]'s [=Document/post-prerendering activation steps list=].
: Otherwise
:: 1. Assert: |platformObject|'s [=relevant global object=] is a {{Window}} object.
:: 1. Return |platformObject|'s [=relevant global object=]'s [=associated Document=]'s [=Document/post-prerendering activation steps list=].
</dl>
Every {{Document}} has an <dfn for="Document">activation start time</dfn>, which is initially a {{DOMHighResTimeStamp}} with a time value of zero.
<p class="note">This is used to keep the time immediately after [=Prompt to unload|prompting to unload=] the [=active document=] of the previous [=browsing context=].
<div algorithm="User-agent initiated prerendering">
[=User agents=] may choose to initiate prerendering without a predecessor document, for example as a result of the address bar or other browser user interactions.
To <dfn export>start user-agent initiated prerendering</dfn> given a [=URL=] |url|:
1. [=Assert=]: |startingURL|'s [=url/scheme=] is an [=HTTP(S) scheme=].
1. Let |bc| be the result of [=creating a new top-level browsing context=].
1. Set |bc|'s [=browsing context/loading mode=] to "`prerender`".
1. Let |request| be a new [=request=] whose [=request/URL=] is |startingURL| and [=request/referrer policy=] is "`no-referrer`".
1. [=Navigate=] |bc| to |request|.
1. When the user indicates they wish to commit the navigation:
1. Let |unsafe time| be the [=unsafe shared current time=].
1. Update the user agent's user interface to present |bc|, e.g., by updating the tab/window contents and the browser chrome.
1. [=prerendering browsing context/Finalize activation=] for |bc| given |url|'s [=url/origin=] and |unsafe time|.
<p class="note">An example of indicating commitment is pressing the Enter key in the URL bar</p>
<p class="note">The user might never indicate such a commitment, or might take long enough to do
so that the user agent needs to reclaim the resources used by the prerender for some more
immediate task. In that case the user agent can [=discard=] |bc|.</p>
</div>
<div algorithm="create a prerendering browsing context">
To <dfn export>create a prerendering browsing context</dfn> given a [=URL=] |startingURL| and a {{Document}} |referrerDoc|:
1. [=Assert=]: |startingURL|'s [=url/scheme=] is an [=HTTP(S) scheme=].
1. If |referrerDoc|'s [=Document/browsing context=] is not a [=top-level browsing context=], then return.
<p class="note">Currently, prerendering from inside a [=nested browsing context=] is not specified or implemented. Doing so would involve tricky considerations around how the prerendered browsing context appears in the browsing context tree.
1. If |referrerDoc|'s [=Document/browsing context=] is an [=auxiliary browsing context=], then return.
<p class="note">This avoids having to deal with any potential opener relationship between the prerendering browsing context and |referrerDoc|'s [=Document/browsing context=]'s [=opener browsing context=].
1. If |referrerDoc|'s [=Document/origin=] is not [=/same site=] with |startingURL|'s [=url/origin=], then return.
<p class="note">Currently, cross-site prerendering is not specified or implemented, although we have various ideas about how it could work in this repository's explainers.
1. If |referrerDoc|'s [=Document/prerendering browsing contexts map=][|startingURL|] [=map/exists=], then return.
1. Let |bc| be the result of [=creating a new top-level browsing context=].
1. Set |bc|'s [=browsing context/loading mode=] to "`prerender`".
1. Set |referrerDoc|'s [=Document/prerendering browsing contexts map=][|startingURL|] to |bc|.
1. Set |bc|'s [=prerendering browsing context/remove from initiator=] to be an algorithm which [=map/removes=] |referrerDoc|[|startingURL|].
<p class="note">As with regular [=browsing contexts=], the [=prerendering browsing context=] can be [=discarded=] for any reason, for example if it becomes unresponsive, performs a restricted operation, or if the user agent believes prerendering takes too much resources.
1. [=Navigate=] |bc| to |startingURL| with the [=source browsing context=] set to |referrerDoc|'s [=Document/browsing context=].
</div>
<div algorithm>
To <dfn for="prerendering browsing context">activate</dfn> a [=prerendering browsing context=] |successorBC| in place of a [=top-level browsing context=] |predecessorBC| given a [=history handling behavior=] |historyHandling|:
1. Assert: |historyHandling| is either "`default`" or "`replace`".
1. Assert: |successorBC| is not [=prerendering browsing context/empty=].
1. Assert: |predecessorBC| is a [=top-level browsing context=].
<!-- The following except for the step that sets the |activation start time| are copied from the navigate algorithm, and probably could benefit from some refactoring to deduplicate. -->
1. Cancel any preexisting but not yet [=navigate/mature=] attempts to navigate |predecessorBC|, including canceling any instances of the [=fetch=] algorithm started by those attempts. If one of those attempts has already <a lt="create and initialize a Document object">created and initialized a new `Document` object</a>, [=Document/abort=] that {{Document}} also.
1. [=Prompt to unload=] the [=active document=] of |predecessorBC|. If the user [=refused to allow the document to be unloaded=], then return.
1. Let |unsafe time| be the [=unsafe shared current time=].
1. [=Document/Abort=] the [=active document=] of |predecessorBC|.
<!-- End copied section. -->
1. <p class="XXX">Appropriately merge the session histories of |predecessorBC| and |successorBC|, respecting |historyHandling|.</p>
This step cannot be specified precisely given the current sorry state of [[HTML]]'s session history specification. Once the <a href="https://github.com/whatwg/html/pull/6315">navigation and session history rewrite</a> pull request is merged, we can make this precise (although we will also have to do several updates to other parts of this prerendering specification).
In the meantime, let's be clear about the intent of the eventual precise-spec, in order to ensure interoperability:
- |successorBC| and |predecessorBC| will still be separate browsing contexts. Before activation, they are each in separate <a href="https://whatpr.org/html/6315/history.html#traversable-navigable">traversable navigables</a> (a new concept introduced in the rewrite). Call those |successorTraversable| and |predecessorTraversable|. Both of these traversable navigables have their own session history list; |successorTraversable|'s will necessarily consist of a single session history entry.
- During activation, we will:
- Move |successorBC| from |successorTraversable| into |predecessorTraversable|.
- Move the session history entry |successorEntry| from |successorTraversable| into |predecessorTraversable|, as follows:
- If |historyHandling| is "`replace`", replace the currently-active session history entry of |predecessorTraversable| with |successorEntry|.
- If |historyHandling| is "`default`", clear the forward session history of |predecessorTraversable|, then append |successorEntry| to |predecessorTraversable|'s session history list and make it the active session history entry.
(Note that as a side effect, this makes |successorBC| the active browsing context of |predecessorTraversable|).
- |successorTraversable| will now be empty and can be unobservably destroyed.
- This setup means that traversing across the associated history entries will cause a browsing context group switch, similar to what currently happens for \`<a http-header>`Cross-Origin-Opener-Policy`</a>\`. Importantly, this will sever any opener relationships, in the same way as COOP. Such traversal includes both the actual activation process above, where we make |successorEntry| active and thus switch from |predecessorBC| to |successorBC|, but also e.g. the user pressing the back button (or the page using `history.back()`) after activation, which will switch from |successorBC| to |predecessorBC|.
1. [=In parallel=]:
1. Update the user agent's user interface to replace |predecessorBC| with |successorBC|, e.g., by updating the tab/window contents and the browser chrome.
1. [=prerendering browsing context/Finalize activation=] for |successorBC| given |predecessorBC|'s [=active document=]'s [=Document/origin=] and |unsafe time|.
</div>
<div algorithm>
To <dfn for="prerendering browsing context">finalize activation</dfn> of a [=prerendering browsing context=] |bc| given an [=origin=] |origin| and a {{DOMHighResTimeStamp}} |unsafe time|:
1. Let |inclusiveDescendants| be « |bc| » [=list/extended=] with |bc|'s [=active document=]'s <a spec="HTML">list of the descendant browsing contexts</a>.
<!-- TODO is this the right task source? Should we make a new one? -->
1. [=list/For each=] |bc| of |inclusiveDescendants|, [=queue a global task=] on the [=networking task source=], given |bc|'s [=browsing context/active window=], to perform the following steps:
1. Set |bc|'s [=browsing context/loading mode=] to "`default`".
1. Let |doc| be |bc|'s [=active document=].
1. If |doc|'s [=Document/origin=] is the same as |origin|, then set |doc|'s [=Document/activation start time=] to the [=relative high resolution time=] for |unsafe time| and |doc|.
1. [=map/For each=] |origin| → |hintSet| in |bc|'s [=prerendering browsing context/prerender-scoped Accept-CH cache=]:
1. [=Add a new Accept-CH cache entry=] to the [=Accept-CH cache=] with the |origin| and |hintSet|.
1. [=Fire an event=] named {{Document/prerenderingchange}} at |doc|.
1. [=list/For each=] |steps| in |doc|'s [=Document/post-prerendering activation steps list=]:
1. Run |steps|.
<p class="note">These steps might return something, like a {{Promise}}. That is just an artifact of how the spec is modified; such return values can always be ignored.
1. Assert: running |steps| did not throw an exception.
<p class="note">The order here is observable for [=browsing contexts=] that share an [=event loop=], but not for those in separate event loops.
</div>
<div algorithm>
A [=prerendering browsing context=] has an associated <dfn for="prerendering browsing context">remove from initiator</dfn>, which is null or an algorithm with no arguments, initially set to null.
To ensure that the references for a [=prerendering browsing context=] are cleared once it is [=discard|discarded=], modify <a spec=HTML for="browsing context">remove</a> by appending the following step:
1. If <var ignore>browsingContext</var> is a [=prerendering browsing context=] and <var ignore>browsingContext</var>'s [=prerendering browsing context/remove from initiator=] is not null, then call <var ignore>browsingContext</var>'s [=prerendering browsing context/remove from initiator=].
</div>
<h3 id="creating-bcs-patch">Modifications to creating browsing contexts</h3>
<div algorithm="create a new nested browsing context patch">
To ensure that any [=nested browsing contexts=] inherit their parent's [=browsing context/loading mode=], modify <a spec=HTML>create a new nested browsing context</a> by appending the following step:
1. Set <var ignore>browsingContext</var>'s [=browsing context/loading mode=] to <var ignore>element</var>'s [=Node/node document=]'s [=Document/browsing context=]'s [=browsing context/loading mode=].
</div>
<h2 id="navigation">Navigation and session history</h2>
<h3 id="navigate-activation">Allowing activation in place of navigation</h3>
<div algorithm="can activate a prerender">
We <dfn>can activate a prerender</dfn> given a [=browsing context=] |browsingContext|, a [=history handling behavior=] |historyHandling|, a string |navigationType|, and a [=request=] |request|, if the following steps return true:
1. Return true if all of the following are true:
* |browsingContext| is a [=top-level browsing context=]
* |browsingContext| is not a [=prerendering browsing context=]
* |browsingContext| is not for a [Fenced Frame](https://github.com/shivanigithub/fenced-frame)
<p class="issue">The concept of a fenced frame browsing context is not yet defined but we need to link to it once it exists.</p>
* |historyHandling| is "`default`" or "`replace`"
* |navigationType| is "`other`"
* |request|'s [=request/method=] is \``GET`\`
* |browsingContext|'s [=active document=]'s [=Document/prerendering browsing contexts map=][|request|'s [=request/URL=]] [=map/exists=] and is not [=prerendering browsing context/empty=]
* The result of calling [=should navigation request of type be blocked by content security policy?=] given |request| and <var ignore>navigationType</var> is "`Allowed`".
1. Otherwise, return false.
</div>
Patch the [=navigate=] algorithm to allow the [=prerendering browsing context/activate|activation=] of a [=prerendering browsing context=] in place of a normal navigation as follows:
<div algorithm="navigate activate patch">
In [=navigate=], append the following steps after the fragment navigation handling (currently step 8):
1. If |resource| is a [=request=] and we [=can activate a prerender=] given |browsingContext|, |historyHandling|, <var ignore>navigationType</var>, and |resource|, then:
1. Let |successorBC| be |browsingContext|'s [=active document=]'s [=Document/prerendering browsing contexts map=][|resource|'s [=request/URL=]].
1. Wait until all navigation attempts of |successorBC| have been [=navigate/mature|matured=].
1. If |successorBC| has not been [=discard|discarded=], then:
1. [=prerendering browsing context/Activate=] |successorBC| in place of |browsingContext| given |historyHandling|.
1. Return.
</div>
Navigation redirects can also [=prerendering browsing context/activate=] [=prerendering browsing contexts=]. This is defined in the [[#navigate-fetch-patch]] section.
<h3 id="navigate-fetch-patch">Navigation fetch changes</h3>
<div algorithm="process a navigate fetch patch">
In [=process a navigate fetch=], append the following steps after the first sub-step under "While true:":
1. If we [=can activate a prerender=] given |browsingContext|, |historyHandling|, <var ignore>navigationType</var>, and <var ignore>request</var>, then:
1. [=prerendering browsing context/Activate=] |browsingContext| in place of <var ignore>sourceBrowsingContext</var> given |historyHandling|.
1. Return.
1. If |browsingContext| is a [=prerendering browsing context=] and <var ignore>currentURL</var>'s [=url/origin=] is not [=/same site=] with |incumbentNavigationOrigin|, then:
1. Run the [=environment discarding steps=] for <var ignore>navigationParams</var>'s [=navigation params/reserved environment=].
1. Return.
Additionally, append the following steps toward the end of the algorithm, after the steps which check <var ignore>locationURL</var>:
1. If |browsingContext| is a prerendering browsing context, and <var ignore>responseOrigin</var> is not [=same origin=] with |incumbentNavigationOrigin|, then:
1. Let |loadingModes| be the result of [=getting the supported loading modes=] for |response|.
1. If |loadingModes| does not [=list/contain=] \`<code><a for="Supports-Loading-Mode">credentialed-prerender</a></code>\`, then set |response| to a [=network error=].
<p class="note">This will cause the prerender to fail in the following steps of the algorithm.
<p class="note">In the future we could possibly also allow the `uncredentialed-prerender` token to work here. However, since that is primarily intended for the cross site case, which isn't specified or implemented yet, we don't want to encourage its use in the wild, so for now we specify that prerendering fails if only the `uncredentialed-prerender` token is found.
</div>
<div algorithm="process a navigate response patch">
In <a spec=HTML>process a navigate response</a>, append the following after the step which establishes the value of |failure|, but before the step which uses it to display an error page:
1. If <var ignore>browsingContext</var> is a [=prerendering browsing context=], and any of the following hold:
* |failure| is true;
* |navigationParams|'s [=navigation params/request=] is null;
* |navigationParams|'s [=navigation params/request=]'s [=request/current URL=]'s [=url/scheme=] is not a [=HTTP(S) scheme=];
* |response| does not [=support prefetch=];
<div class="note">Responses which are ineligible for prefetch, e.g. because they have an error status, are not eligible for prerender either. A future version of this specification might allow responses to more clearly delineate the two.</div>
* |response| has a \``Content-Disposition`\` header specifying the `attachment`
disposition type; or
* |response|'s [=response/status=] is 204 or 205,
then:
1. Run the [=environment discarding steps=] for |navigationParams|'s [=navigation params/reserved environment=].
1. Return.
</div>
<div algorithm="process a navigate URL scheme patch">
In <a spec=HTML>process a navigate URL scheme</a>, insert the following step before the step which displays inline content:
1. Otherwise, if <var ignore>browsingContext</var> is a [=prerendering browsing context=], then return.
</div>
<h3 id="delay-crossorigin">Delaying cross-origin nested browsing context navigations</h3>
Patch the [=process a navigate fetch=] algorithm to allow delaying or discarding of cross-origin navigations inside browsing contexts that are [=nested browsing context|nested=] inside a [=prerendering browsing context=] as follows:
<div algorithm="navigate delay cross-origin patch">
In [=process a navigate fetch=], append the following steps after the cross-origin redirect check (currently step 13.1):
1. Let |crossOrigin| be <var ignore>hasCrossOriginRedirects</var>.
1. If |bc|'s <a spec="HTML">container</a> is not null, and |bc|'s <a spec="HTML">container</a>'s [=Node/node document=]'s [=Document/origin=] is not [=same origin=] with <var ignore>currentURL</var>'s [=url/origin=], then set |crossOrigin| to true.
1. If |crossOrigin| is true and |bc|'s [=top-level browsing context=] is a [=prerendering browsing context=], then the user agent must either wait to continue this algorithm until |bc| is [=prerendering browsing context/activate|activated=], or [=discard=] |bc|'s [=top-level browsing context=].
</div>
<h3 id="always-replacement">Maintaining a trivial session history</h3>
<div algorithm="navigate historyHandling patch">
Patch the [=navigate=] algorithm to ensure the [=session history=] of a [=prerendering browsing context=] stays trivial by prepending the following step before all others:
1. If <var ignore>browsingContext</var> is a [=prerendering browsing context=], then:
1. Assert: |historyHandling| is not "`entry update`", since prerendering browsing contexts have trivial session histories and thus will never end up [=traverse the history|traversing=] back to an entry with null [=session history entry/document=].
1. If |historyHandling| is "`default`", then set |historyHandling| to "`replace`".
</div>
<div algorithm="URL and history update steps patch">
Patch the <a spec=HTML>URL and history update steps</a> by adding the following step after step 1:
1. If <var ignore>browsingContext</var> is a [=prerendering browsing context=], then set <var ignore>isPush</var> to false.
</div>
<h3 id="interaction-with-workers">Interaction with Worker Lifetime</h3>
<div algorithm="The worker's lifetime patch">
In <a spec=HTML>The worker's lifetime</a>, modify the definition of [=active needed worker=], to count workers with a [=prerendering browsing context=] owner as not [=active needed worker|active=]:
A worker is said to be an [=active needed worker=] if any of its [=WorkerGlobalScope/owner set|owners=] are either {{Document}} objects that are [=fully active=] and their [=Document/browsing context=] is not a [=prerendering browsing context=], or [=active needed worker|active needed workers=].
<p class="note">This means that the worker's script would load, but the execution would be suspended until the document is activated.
</div>
<h3 id="cleanup-upon-discarding">Cleanup upon discarding a {{Document}}</h3>
Modify the [=discard a document|discard=] algorithm for {{Document}}s by appending the following step:
<div algorithm="discard a Document patch">
1. [=list/Empty=] <var ignore>document</var>'s [=Document/post-prerendering activation steps list=].
</div>
<h2 id="interaction-with-other-specs">Interaction with other specifications and concepts</h2>
<h3 id="interaction-with-visibility-state">Interaction with Page Visibility</h3>
Documents in [=prerendering browsing contexts=] always have a [=Document/visibility state=] of "<code>hidden</code>".
<p class="issue">What about portals? Portals might not be hidden, and portals are envisioned to be a type of prerendering browsing context.</p>
<h3 id="interaction-with-system-focus">Interaction with system focus</h3>
[=Prerendering browsing contexts=] never have [=system focus=].
<h3 id="performance-navigation-timing-extension">Extensions to the {{PerformanceNavigationTiming}} interface</h3>
Extend the {{PerformanceNavigationTiming}} interface as follows:
<pre class="idl">
partial interface PerformanceNavigationTiming {
readonly attribute DOMHighResTimeStamp activationStart;
};
</pre>
The <dfn attribute for="PerformanceNavigationTiming">activationStart</dfn> getter steps are:
1. Return a {{DOMHighResTimeStamp}} with a time value equal to the [=current document=]'s [=Document/activation start time=].
<h3 id="client-hint-cache">Interaction with Client Hint Cache</h3>
We need to ensure that the [=Accept-CH cache=], which is owned by user agent as a global storage, is not modified while prerendering, but is properly updated after activation. The following modifications ensure this.
Each [=prerendering browsing context=] has a <dfn for="prerendering browsing context">prerender-scoped Accept-CH cache</dfn>, which is an [=ordered map=] of [=origin=] to [=client hints sets=].
This stores which client hints each origin has opted into receiving, until it can be copied to the global [=Accept-CH cache=] when [=prerendering browsing context/finalize activation|activation is finalized=].
<div algorithm>
To <dfn>add a new prerender Accept-CH cache entry</dfn> given a [=prerendering browsing context=] |bc|, an [=origin=] |origin|, and a [=client hints set=] |hintSet|:
1. Let |cache| be |bc|'s [=prerendering browsing context/prerender-scoped Accept-CH cache=].
1. [=map/set=] |cache|[|origin|] to |hintSet|.
<p class="note">This is similar to [=add a new Accept-CH cache entry=].</p>
</div>
<div algorithm="append client hints to request patch">
Modify the <a spec=CLIENT-HINTS-INFRASTRUCTURE>append client hints to request</a> algorithm, by prepending the following steps before iterating over the |hintSet|:
1. Let |browsingContext| be <var ignore>settingsObject</var>’s [=environment settings object/global object=]'s [=Window/browsing context=].
If |browsingContext| is a [=prerendering browsing context=], then:
1. Let |origin| be <var ignore>request</var>'s [=request/url=]'s [=url/origin=].
1. Set |hintSet| to the [=set/union=] of |hintSet| and |browsingContext|'s [=prerendering browsing context/prerender-scoped Accept-CH cache=][|origin|].
</div>
<div algorithm="update the Client Hints set patch">
Modify the <a spec=CLIENT-HINTS-INFRASTRUCTURE>update the Client Hints set</a> algorithm, by updating the last step.
1. Let |origin| be <var ignore>response</var>'s [=response/url=]'s [=url/origin=].
1. Let |hintSet| be <var ignore>settingsObject</var>'s [=environment settings object/client hints set=].
1. If |browsingContext| is a [=prerendering browsing context=], [=add a new prerender Accept-CH cache entry=] with |browsingContext|, |origin| and |hintSet|.
1. Otherwise, [=add a new Accept-CH cache entry=] with |origin| and |hintSet|.
</div>
<h2 id="supports-loading-mode">The \`<dfn http-header><code>Supports-Loading-Mode</code></dfn>\` HTTP response header</h2>
<em>The following section would be added as a sub-section of [[HTML]]'s <a href="https://html.spec.whatwg.org/multipage/browsers.html#browsers">Loading web pages</a> section.</em>
In some cases, cross-origin web pages might not be prepared to be loaded in a novel context. To allow them to opt in to being loaded in such ways, the \`<a http-header><code>Supports-Loading-Mode</code></a>\` HTTP response header can be used. This header is a [=structured header=]; if present its value must be the single [=structured header/token=] \`<code><a for="Supports-Loading-Mode">credentialed-prerender</a></code>\`.
<p class="note">The parsing is actually done as a [=structured header/list=] of [=structured header/tokens=], and unknown tokens will be ignored. However, authors are best off avoiding any form except the single-token one, for future compatibility.
The \`<code><dfn for="Supports-Loading-Mode">credentialed-prerender</dfn></code>\` token indicates that the response can be used to create a [=prerendering browsing context=], despite the prerendering being initiated by a cross-origin same-site referrer. Without this opt-in, such prerenders will fail, as outlined in [[#navigate-fetch-patch]].
To <dfn>get the supported loading modes</dfn> for a [=response=] |response|:
1. If |response| is a [=network error=], then return an empty list.
1. Let |slmHeader| be the result of [=header list/getting a structured field value=] given \`<a http-header><code>Supports-Loading-Mode</code></a>\` and "`list`" from |response|'s [=response/header list=].
1. Return a [=list=] containing all elements of |slmHeader| that are [=structured header/tokens=].
<h2 id="nonsense-behaviors">Preventing nonsensical behaviors</h2>
Some behaviors might make sense in most [=top-level browsing contexts=], but do not make sense in [=prerendering browsing contexts=]. This section enumerates specification patches to enforce such restrictions.
<h3 id="patch-window-apis">APIs for creating and navigating browsing contexts by name</h3>
Modify the definition of <a spec=HTML>script-closable</a> to prevent window closing while in a [=prerendering browsing context=]:
A [=browsing context=] is <dfn noexport>script-closable</dfn> if either of the following is true:
* it is an [=auxiliary browsing context=] that was created by script (as opposed to by an action of the user); or
* it is a [=top-level browsing context=] <ins>that is not a [=prerendering browsing context=]</ins> and whose [=session history=] contains only one {{Document}}.
<h2 id="intrusive-behaviors">Preventing intrusive behaviors</h2>
Various behaviors are disallowed in [=prerendering browsing contexts=] because they would be intrusive to the user, since the prerendered content is not being actively interacted with.
<h3 id="patch-downloading">Downloading resources</h3>
Modify the <a spec=HTML>download the hyperlink</a> algorithm to ensure that downloads inside [=prerendering browsing contexts=] are delayed until [=prerendering browsing context/activate|activation=], by inserting the following before the step which goes [=in parallel=]:
<div algorithm="download the hyperlink patch">
1. If <var ignore>subject</var>'s [=Node/node document=]'s [=Document/browsing context=] is a [=prerendering browsing context=], then append the following step to <var ignore>subject</var>'s [=platform object/post-prerendering activation steps list=] and return.
</div>
<h3 id="patch-modals">User prompts</h3>
<div algorithm="cannot show simple dialogs patch">
Modify the <a spec=HTML>cannot show simple dialogs</a> algorithm, given a {{Window}} |window|, by prepending the following step:
1. If |window|'s [=Window/browsing context=] is a [=prerendering browsing context=], then return true.
</div>
<div algorithm="window print() patch">
Modify the {{Window/print()}} method steps by prepending the following step:
1. If [=this=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then return.
</div>
<h3 id="delay-async-apis">Delaying async API results</h3>
Many specifications need to be patched so that, if a given algorithm invoked in a [=prerendering browsing context=], most of its work is deferred until the browsing context is [=prerendering browsing context/activated=]. This is tricky to do uniformly, as many of these specifications do not have great hygeine around <a href="https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-for-spec-authors">using the event loop</a>. Nevertheless, the following sections give our best attempt.
<h4 id="delay-while-prerendering">The {{[DelayWhilePrerendering]}} extended attribute</h4>
To abstract away some of the boilerplate involved in delaying the action of asynchronous methods until [=prerendering browsing context/activated|activation=], we introduce the <dfn extended-attribute>[DelayWhilePrerendering]</dfn> Web IDL extended attribute. It indicates that when a given method is called in a [=prerendering browsing context=], it will immediately return a pending promise and do nothing else. Only upon activation will the usual method steps take place, with their result being used to resolve or reject the previously-returned promise.
The {{[DelayWhilePrerendering]}} extended attribute must take no arguments, and must only appear on a <a lt="regular operation" spec="WEBIDL">regular</a> or <a spec="WEBIDL">static operation</a> whose <a spec="WEBIDL">return type</a> is a <a spec="WEBIDL">promise type</a> or {{undefined}}, and whose <a spec="WEBIDL">exposure set</a> contains only {{Window}}.
<div algorithm="DelayWhilePrerendering method steps">
The method steps for any operation annotated with the {{[DelayWhilePrerendering]}} extended attribute are replaced with the following:
1. Let |realm| be the [=current Realm=].
1. If the operation in question is a <a spec="WEBIDL">regular operation</a>, then set |realm| to the [=relevant Realm=] of [=this=].
1. If |realm|'s [=Realm/global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then:
1. Let |promise| be [=a new promise=], created in |realm|.
1. Append the following steps to [=this=]'s [=platform object/post-prerendering activation steps list=]:
1. Let |result| be the result of running the originally-specified steps for this operation, with the same [=this=] and arguments.
1. [=Resolve=] |promise| with |result|.
1. If this operation's <a spec="WEBIDL">return type</a> is a <a spec="WEBIDL">promise type</a>, then return |promise|.
1. Otherwise, return the result of running the originally-specified steps for this operation, with the same [=this=] and arguments.
</div>
<h4 id="patch-service-workers">Service Workers</h3>
Add {{[DelayWhilePrerendering]}} to {{ServiceWorkerRegistration/update()}}, {{ServiceWorkerRegistration/unregister()}}, {{ServiceWorkerContainer/register(scriptURL, options)}}, {{ServiceWorker/postMessage(message, transfer)}}, and {{ServiceWorker/ postMessage(message, options)}}.
<p class="note">This allows prerendered page to take advantage of existing service workers, but not have any effect on the state of service worker registrations.</p>
<h4 id="patch-broadcast-channel">BroadcastChannel</h4>
Add {{[DelayWhilePrerendering]}} to {{BroadcastChannel/postMessage()}}.
<h4 id="patch-geolocation">Geolocation API</h4>
<div algorithm="Geolocation getCurrentPosition patch">
Modify the {{Geolocation/getCurrentPosition()}} method steps by prepending the following step:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then append the following steps to [=this=]'s [=platform object/post-prerendering activation steps list=] and return.
</div>
<div algorithm="Geolocation watchPosition patch">
Modify the {{Geolocation/watchPosition()}} method steps by prepending the following step:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then:
1. Let |watchProcessId| be a new unique <a spec=GEOLOCATION>watch process</a> ID, and note it as a <dfn>post-prerendering activation geolocation watch process ID</dfn>.
1. Append the following steps to [=this=]'s [=platform object/post-prerendering activation steps list=], with the modification that the <a spec=GEOLOCATION>watch process</a> generated must use |watchProcessId| as its ID, and return |watchProcessId|.
</div>
<div algorithm="Geolocation clearWatch patch">
Modify the {{Geolocation/clearWatch(watchId)}} method steps by prepending the following step:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then:
1. If <var ignore>watchId</var> is a [=post-prerendering activation geolocation watch process ID=], then remove its corresponding steps from [=this=]'s [=platform object/post-prerendering activation steps list=].
1. Return.
</div>
<h4 id="patch-serial">Web Serial API</h4>
Add {{[DelayWhilePrerendering]}} to {{Serial/requestPort()}}.
TODO: the below could probably be done by generalizing {{[DelayWhilePrerendering]}} to use owner document while in dedicated workers.
<div algorithm="Serial getPorts patch">
Modify the {{Serial/getPorts()}} method steps by inserting the following steps after the initial creation of |promise|:
<!-- TODO link owner document once https://github.com/whatwg/html/pull/6379 lands -->
1. Let |document| be [=this=]'s [=relevant global object=]'s [=associated Document=], if [=this=]'s [=relevant global object=] is a {{Window}}, or [=this=]'s [=relevant global object=]'s owner document, if [=this=]'s [=relevant global object=] is a {{DedicatedWorkerGlobalScope}}.
1. If |document| is null, then return [=a promise rejected with=] a "{{SecurityError}}" {{DOMException}}.
1. If |document|'s [=Document/browsing context=] is a [=prerendering browsing context=], then append the following steps to |document|'s [=Document/post-prerendering activation steps list=] and return |promise|.
</div>
<h4 id="patch-notifications">Notifications API</h4>
Add {{[DelayWhilePrerendering]}} to {{Notification/requestPermission()}}.
<div algorithm="Notification constructor patch">
Modify the {{Notification/Notification()}} constructor steps by replacing the step which goes [=in parallel=] with the following:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then append these steps to [=this=]'s [=platform object/post-prerendering activation steps list=]. Otherwise, run these steps [=in parallel=].
</div>
<div algorithm="Notification permission patch">
Modify the {{Notification/permission}} static getter steps by replacing them with the following:
1. If the [=current global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then return "`default`".
<p class="note">This allows implementations to avoid looking up the actual permission state, which might not be synchronously accessible especially in the case of [=prerendering browsing contexts=]. Web developers can then call {{Notification/requestPermission()|Notification.requestPermission()}}, which per the above modifications will only actually do anything after activation. At that time we might discover that the permission is "`granted`" or "`denied`", so the browser might not actually ask the user like would normally be the case with "`default`". But that's OK: it's not observable to web developer code.</p>
1. Otherwise, <a spec="NOTIFICATIONS">get the notifications permission state</a> and return it.
</div>
<h4 id="patch-midi">Web MIDI API</h4>
Add {{[DelayWhilePrerendering]}} to {{Navigator/requestMIDIAccess()}}.
<h4 id="patch-idle-detection">Idle Detection API</h4>
Add {{[DelayWhilePrerendering]}} to {{IdleDetector/start()}}.
<p class="note">The other interesting method, {{IdleDetector/requestPermission()|IdleDetector.requestPermission()}}, is gated on [=transient activation=]. However, even if permission was previously granted for the origin in question, we delay starting any idle detectors while prerendering.
<h4 id="patch-generic-sensor">Generic Sensor API</h4>
<div algorithm="Sensor start patch">
Modify the {{Sensor/start()}} method steps by inserting the following steps after the state is set to "`activating`":
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then append the following steps to [=this=]'s [=platform object/post-prerendering activation steps list=] and return.
1. If [=this=].{{Sensor/[[state]]}} is "`idle`", then return.
<p class="note">This ensures that if this portion of the algorithm was delayed due to prerendering, and in the meantime {{Sensor/stop()}} was called, we do nothing upon activating the prerender.
1. Assert: [=this=].{{Sensor/[[state]]}} is "`activating`".
</div>
<h4 id="patch-web-nfc">Web NFC</h4>
Add {{[DelayWhilePrerendering]}} to {{NDEFReader/write()}} and {{NDEFReader/scan()}}.
<h4 id="patch-battery">Battery Status API</h4>
<div algorithm="Navigator getBattery patch">
Modify the {{Navigator/getBattery()}} method steps by prepending the following step:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then append the following steps to [=this=]'s [=platform object/post-prerendering activation steps list=] and return [=this=]'s [=battery promise=].
</div>
<h4 id="patch-orientation-lock">Screen Orientation API</h4>
<div algorithm="apply an orientation lock patch">
Modify the [=apply an orientation lock=] algorithm steps by overwriting the first few steps, before it goes [=in parallel=], as follows:
1. Let |promise| be a new promise.
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then append the following steps to [=this=]'s [=platform object/post-prerendering activation steps list=] and return |promise|.
1. If the [=user agent=] does not support locking the screen orientation, then [=reject=] |promise| with a "{{NotSupportedError}}" {{DOMException}} and return |promise|.
1. If the [=document=]'s [=Document/active sandboxing flag set=] has the [=sandboxed orientation lock browsing context flag=] set, or the [=user agent=] doesn't meet the [=pre-lock conditions=] to perform an orientation change, then [=reject=] |promise| with a "{{SecurityError}}" {{DOMException}} and return |promise|.
1. Set the [=document=]'s \[[orientationPendingPromise]] to |promise|.
</div>
Add {{[DelayWhilePrerendering]}} to {{ScreenOrientation/unlock()}}.
<p class="note">This latter modification is necessary to ensure that code that calls {{ScreenOrientation/lock()|screen.orientation.lock()}} followed by {{ScreenOrientation/unlock()|screen.orientation.unlock()}} produces the expected results.
<h4 id="patch-gamepads">Gamepad</h4>
<div algorithm="getGamepads patch">
Modify the {{Navigator/getGamepads()}} method steps by prepending the following step:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then return an empty sequence.
</div>
<div algorithm="gamepad events patch">
Modify the discussion of the {{Window/gamepadconnected}} and {{Window/gamepaddisconnected}} events to specify that the user agent must not dispatch these events if the {{Window}} object's [=Window/browsing context=] is a [=prerendering browsing context=].
</div>
<div algorithm="gamepadconnected post-prerendering">
Modify the {{Window/gamepadconnected}} section to indicate that every {{Document}} |document|'s [=Document/post-prerendering activation steps list=] should gain the following steps:
1. If |document| is [=allowed to use=] the "`gamepad`" feature, and |document|'s [=relevant settings object=] is a [=secure context=], and any gamepads are connected, then for each connected gamepad, [=fire an event=] named {{Window/gamepadconnected}} at |document|'s [=relevant global object=] using {{GamepadEvent}}, with its {{GamepadEvent/gamepad}} attribute initialized to a new {{Gamepad}} object representing the connected gamepad.
</div>
<h4 id="eme-patch">Encrypted Media Extensions</h4>
Add {{[DelayWhilePrerendering]}} to {{Navigator/requestMediaKeySystemAccess()}}.
<h4 id="autoplay-patch">Media Autoplay</h4>
Modify the [=playing the media resource=] section to indicate that the the [=current playback position=] of a {{HTMLMediaElement}} must increase monotonically only when the {{Document}} is not {{Document/prerendering}}.
<h4 id="media-capture-patch">Media Capture and Streams</h4>
Add {{[DelayWhilePrerendering]}} to {{Navigator/getUserMedia()}}, {{MediaDevices/getUserMedia()}} and {{MediaDevices/enumerateDevices()}}.
<div algorithm="mediacapture device-change path">
Modify the {{MediaDevices}} section by prepending the following step to the [=device change notification steps=]:
1. If [=this=]'s [=relevant global object=]'s [=Window/browsing context=] is a [=prerendering browsing context=], then return.
</div>
<h4 id="audio-output-patch">Audio Output Devices API</h4>
Add {{[DelayWhilePrerendering]}} to {{MediaDevices/selectAudioOutput()}}.
<h4 id="push-patch">Push API</h4>
Add {{[DelayWhilePrerendering]}} to {{PushManager/subscribe()}}.
<h4 id="background-fetch-patch">Background Fetch</h4>
Add {{[DelayWhilePrerendering]}} to {{BackgroundFetchManager/fetch()}}.
<h4 id="persist-patch">Storage API</h4>
Add {{[DelayWhilePrerendering]}} to {{StorageManager/persist()}}.
<h4 id="screen-capture-patch">Screen Capture</h4>
Add {{[DelayWhilePrerendering]}} to {{MediaDevices/getDisplayMedia()}}.
<h4 id="webusb-patch">WebUSB API</h4>
Add {{[DelayWhilePrerendering]}} to {{USB/getDevices()}} and {{USB/requestDevice()}}.
<h4 id="web-bluetooth-patch">Web Bluetooth</h4>
Add {{[DelayWhilePrerendering]}} to {{Bluetooth/getDevices()}} and {{Bluetooth/requestDevice()}}.
<h4 id="webhid-patch">WebHID API</h4>
Add {{[DelayWhilePrerendering]}} to {{HID/getDevices()}} and {{HID/requestDevice()}}.
<h4 id="webxr-patch">WebXR Device API</h4>
Add {{[DelayWhilePrerendering]}} to {{XRSystem/requestSession()}}.