This specification defines an API extending the HTMLMediaElement that enables controlling remote playback of media from a web page.

This document builds on the group's experience on presenting web content on external presentation-type displays, and re-uses patterns and design considerations from the Presentation API specification whenever appropriate [[PRESENTATION-API]].

Although this document is still a work in progress and is subject to change, the Working Group believes that the API surface is stable. Most of the remaining issues listed on the issue tracker are considered minor at this stage except for Issue #41.

Issue #41 discusses the set of media playback features that remote playback devices are expected to support. The group will seek further developer feedback and implementation experience to identify any interoperability issues around these features when used during remote playback, and will further clarify the specification based on feedback received.

For other issues or concerns, it is possible to file a bug or send an email to the mailing list. For small editorial changes like typos, sending a pull request is appreciated.

The Working Group invites everyone to review this document, and will work with relevant groups at W3C to conduct horizontal reviews on accessibility, internationalization, privacy, security and technical architecture principles.

No feature has been identified as being at risk.

The Second Screen Working Group will develop a test suite for the Remote Playback API during the Candidate Recommendation period and prepare an implementation report. For this specification to advance to Proposed Recommendation, two independent, interoperable implementations of each feature must be demonstrated, as detailed in the Candidate Recommendation exit criteria section.

This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.

Implementations that use ECMAScript to expose the APIs defined in this specification MUST implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]].

Introduction

This specification aims to make remote playback devices such as connected TVs, projectors or audio-only speakers, available to the Web and takes into account playback devices that are attached using wired (HDMI, DVI, or similar) and wireless technologies (Miracast, Chromecast, DLNA, AirPlay, or similar).

Devices with limited screen size or quiet speakers lack the ability to playback media content to a larger audience, for example, a group of colleagues in a conference room, or friends and family at home. Playing media content on an external larger and/or louder remote playback device helps to improve the perceived quality and impact of the played media.

At its core, this specification enables a page that acts as the browsing context to initiate and control remote playback of a particular media element on a selected remote playback device. How the remoting is initiated and controlled is left to the UA in order to allow the use of remote playback devices that can be attached in a wide variety of ways. For example, when a remote playback device is attached using HDMI or Miracast, the same UA that acts as the browsing context renders the remote media. Instead of playing the media on the same device, however, it can use whatever means the operating system provides for using the external remote playback device. In such a case, both the browsing context and the media player run on the same UA and the operating system is used to route the player output to the remote playback device. This is commonly referred to as the media mirroring case. This specification imposes no requirements on the remote playback devices connected in such a manner.

If the remote playback device is able to play the media and communicate with the browsing context but is unable to fetch the media, the browsing context needs to fetch the media data and pass it on to the remote playback device for rendering. This is commonly referred to as media remoting case.

If the remote playback device is able to fetch and play the media and communicate with the browsing context, the browsing context does not need to fetch or render the remoted media. In this case, the UA acts as a proxy that requests the remote playback device to play the media itself by passing the necessary data like the media source. This is commonly referred to as the media flinging case. This way of attaching to displays could be enhanced in the future by defining a standard protocol for delivering these types of messages that remote playback devices could choose to implement.

The API defined here is intended to be used with UAs that attach to remote playback device devices through any of the above means.

Use cases and requirements

The use cases and requirements of this specification are captured in a separate document available here.

Dependencies

The following concepts and interfaces are defined in [[!HTML]]:

The term URL is defined in the WHATWG URL standard [[!URL]].

The term throw in this specification is used as defined in [[!WEBIDL]].

The interface PresentationRequest is defined in [[PRESENTATION-API]].

The term potentially trustworthy origin is defined in [[!SECURE-CONTEXTS]].

The following exception names are defined by [[!WEBIDL]] and used by this specification:

Examples

This section shows code examples that highlight the usage of the main features of the Remote Playback API. In these examples, player.html implements the player page controlling the remote playback and media.ext is the media file to be played remotely. Both the page and the media are served from the domain https://example.org. Please refer to the comments in the code examples for further details.

Monitor availability of remote playback devices example 

<!-- player.html -->
<!-- The video element with custom controls that supports remote playback. -->
<video id="videoElement" src="https://example.org/media.ext" />
<button id="deviceBtn" style="display: none;">Pick device</button>
<script>
  // The "Pick device" button is visible if at least one remote playback device is available.
  var deviceBtn = document.getElementById("deviceBtn");
  var videoElem = document.getElementById("videoElement");

  function availabilityCallback(available) {
    // Show or hide the device picker button depending on device availability.
    deviceBtn.style.display = available ? "inline" : "none";
  }

  videoElem.remote.watchAvailability(availabilityCallback).catch(function() {
    // Availability monitoring is not supported by the platform, so discovery of
    // remote playback devices will happen only after remote.prompt() is called.
    // Pretend the devices are available for simplicity; or, one could implement
    // a third state for the button.
    deviceBtn.style.display = "inline";
  });
</script>

Starting remote playback of a video example 

<!-- player.html -->
<script>
  devicesBtn.onclick = function() {
    // Request the user to select a remote playback device.
    videoElem.remote.prompt()
      // Update the UI and monitor the connected state.
      .then(updateRemotePlaybackState);
      // Otherwise, the user cancelled the selection UI or no screens were found.
  };
<script>

Monitoring remote playback state changes 

<!-- player.html -->
<script>
  // The remote playback may be initiated by the user agent,
  // so check the initial state to sync the UI with it.
  if (videoElem.remote.state == "disconnected")
    switchToLocalUI();
  else
    switchToRemoteUI();

  videoElem.remote.onconnecting = switchToRemoteUI;
  videoElem.remote.onconnect = swithToRemoteUI;
  videoElem.remote.ondisconnect = switchToLocalUI;

  // Handles both 'connecting' and 'connected' state. Calling more than once
  // is a no-op.
  function switchToRemoteUI() {
    // Indicate that the state is 'connecting' or 'connected' to the user.
    // For example, hide the video element as only controls are needed.
    videoElem.style.display = "none";

    // Stop monitoring the availability of remote playback devices.
    videoElem.remote.cancelWatchAvailability();
  };

  function switchToLocalUI() {
    // Show the video element.
    videoElem.style.display = "inline";
    // Start watching the device availability again.
    videoElem.remote.watchAvailability(availabilityCallback);
  };
<script>

API

Common idioms

A local playback device is the device the browsing context is running on along with the default video/audio outputs the device has.

A local playback device might have extra outputs, like an external display or speakers/headphones. As long as the switch of what output to use happens outside of the user agent on the system level, the playback is considered to happen on a local playback device for the purpose of this spec.

A remote playback device is any other device but the local playback device that the browsing context can use to play media on.

A media element state is the set of all single media element properties observable by the page and/or the user via the user agent implementation. The new properties introduced by this spec are not considered part of the media element state for convenience.

For example, the paused attribute or the pause/resume button reflecting that state on the default controls of the media element would be a part of media element state.

A local playback state is the user agent implementation of media element state for the particular media element for playback on the local playback device.

A remote playback state is the user agent implementation of media element state for the particular media element for playback on the certain remote playback device.

For a good user experience it is important that the media element state doesn't change unexpectedly when the state changes. It is also important that remote playback state is in sync with the media element state so when the media is paused on the remote playback device it looks paused to both the user and the page.

RemotePlayback interface 

          [Exposed=Window]
          interface RemotePlayback : EventTarget {
            Promise<long> watchAvailability(RemotePlaybackAvailabilityCallback callback);
            Promise<void> cancelWatchAvailability(optional long id);

            readonly attribute RemotePlaybackState state;

            attribute EventHandler onconnecting;
            attribute EventHandler onconnect;
            attribute EventHandler ondisconnect;

            Promise<void> prompt();
          };

          enum RemotePlaybackState {
            "connecting",
            "connected",
            "disconnected"
          };

          callback RemotePlaybackAvailabilityCallback = void(boolean available);

A RemotePlayback object is an interface allowing the page to detect availability of, connect to and control playback on remote playback devices.

A RemotePlaybackState enumeration represents the state of connection to some remote playback device.

A RemotePlaybackAvailabilityCallback represents the remote playback device availability information.

Observing remote playback device availability

A RemotePlaybackAvailabilityCallback is the way for the page to obtain the remote playback device availability for the corresponding media element. If the user agent can monitor the list of available remote playback devices in the background (without a pending request to prompt()), the RemotePlaybackAvailabilityCallback behavior defined below MUST be implemented by the user agent. Otherwise, the promise returned by watchAvailability() MUST be rejected with NotSupportedError.

The set of availability callbacks

The user agent MUST keep track of the set of availability callbacks registered with each media element through the watchAvailability() method. The set of availability callbacks for each RemotePlayback object is represented as a set of tuples (callbackId, callback), initially empty, where:

  1. callbackId is a positive integer unique among all ids returned by watchAvailability() in a given browsing context;
  2. callback is a RemotePlaybackAvailabilityCallback object.

Since there's one and only one RemotePlayback object per each media element, set of availability callbacks of a media element is the same set as the set of availability callbacks of the RemotePlayback object referred to by the element's remote property.

The combined set of all sets of availability callbacks of all RemotePlayback objects known to the browsing context is referred to as global set of availability callbacks.

The list of available remote playback devices

The user agent MUST keep a list of available remote playback devices. This list contains remote playback devices and is populated based on an implementation specific discovery mechanism. It is set to the most recent result of the algorithm to monitor the list of available remote playback devices or an empty list if the algorithm hasn't been run yet.

The user agent MAY not support running the algorithm to monitor the list of available remote playback devices continuously, for example, because of platform or power consumption restrictions. In this case the promise returned by watchAvailability() MUST be rejected with NotSupportedError, the global set of availability callbacks will be empty and the algorithm to monitor the list of available remote playback devices will only run as part of the initiate remote playback algorithm.

When the global set of availability callbacks is not empty, the user agent MUST monitor the list of available remote playback devices continuously, so that pages can keep track of the last value received via the registered callbacks to offer remote playback only when there are available devices.

The user agent is expected not to monitor the list of available remote playback devices when possible, to satisfy the power saving non-functional requirement. For example, the user agent might choose not to run the monitoring algorithm when the global set of availability callbacks is empty, when every page that has media elements with non-empty set of availability callbacks is in the background.

Some remote playback devices may only be able to play a subset of media resources because of functional, security or hardware limitations. Examples are set-top boxes, smart TVs or networked speakers capable of rendering only certain formats of video and/or audio. We say that such a device is a compatible remote playback device for a media resource if the user agent can reasonably guarantee that the remote playback of the media specified by the resource will succeed on that device.

The user agent can use the srclang attribute of the track element as a hint of the language of the text track data to help identify a compatible remote playback device.

The media resources of a media element, that were considered by the user agent to find a compatible remote playback device, are called the availability sources set.

The media resource of a media element, that is used to initiate remote playback on the selected remote playback device is called remote playback source. Remote playback source MUST belong to availability sources set.

The mechanism of picking the availability sources set and the remote playback source is implementation-specific. For example, the user agent MUST either use the currentSrc of the media element for both availability monitoring and remote playback or use all the media resources associated with the media element as the availability sources set and pick one of the resources as the remote playback source after user selects the remote playback device.

Remote playback is said to be unavailable for the media element if the list of available remote playback devices is empty or none of them is compatible with any source from availability sources set for the media element. The remote playback is said to be available otherwise. A boolean set to false if the remote playback is unavailable for the media element or true if it is available is called availability for the media element.

If the user agent stops monitoring the list of available remote playback devices (for example by a user control or for power saving), it SHOULD invoke all callbacks in the global set of availability callbacks with false so that pages can update their user experience appropriately. It SHOULD also set the availability value for all media elements to false so that availability information can be propagated correctly if the user agent later resumes monitoring the list of available remote playback devices.

Getting the remote playback devices availability information

When the watchAvailability() method is called, the user agent MUST run the following steps:

Input
callback, the callback that will get fired with availability information.
Output
promise, a promise.
  1. Let promise be a new promise.
  2. Return promise, and run the following steps below:
  3. If the disableRemotePlayback attribute is present for the media element, reject the promise with InvalidStateError and abort all the remaining steps.
  4. If the user agent is unable to monitor the list of available remote playback devices for the entire lifetime of the browsing context (for instance, because the user has disabled this feature), then run the following steps in parallel:
    1. Fulfill promise.
    2. Queue a task to invoke the callback with false as its argument.
    3. Abort all remaining steps.
  5. If the user agent is unable to continuously monitor the list of available remote playback devices but can do it for a short period of time when initiating remote playback, then:
    1. Reject promise with a NotSupportedError exception.
    2. Abort all remaining steps.
  6. Let callbackId be a positive integer unique among all the callbackIds previously returned by these steps in the browsing context of the media element.
  7. Create a tuple (callbackId, callback) and add it to the set of availability callbacks for this media element.
  8. Fulfill promise with the callbackId and run the following steps in parallel:
    1. Queue a task to invoke the callback with the current availability for the media element.
    2. If the user agent is not monitoring the list of available remote playback devices, run the algorithm to monitor the list of available remote playback devices.

A simple algorithm for assigning callbackId values is to keep a counter for each browsing context and incrementing it in step 6.

To avoid leaking information that could fingerprint the user, the user agent is expected not to assign a callbackId that uses any persistent information from the browser profile or a remote playback device.
Monitoring the list of available remote playback devices

If the set of availability callbacks is non-empty, or there is a pending request to initiate remote playback, the user agent MUST monitor the list of available remote playback devices by running the following steps:

  1. Retrieve available remote playback devices (using an implementation specific mechanism) and let newDevices be this list.
  2. For each media element known to the browsing context:
    1. If the disableRemotePlayback attribute is present for mediaElement, abort all the remaining steps for this tuple and continue to the next one.
    2. Set newAvailabilityValue to the value of availability for the media element calculated using the newDevices list instead of the list of available remote playback devices.
    3. If the current availability is not equal to newAvailabilityValue, then for each (callbackId, callback) of the element's set of availability callbacks:
      1. Queue a task to invoke callback with newAvailabilityValue as its argument.
  3. Set the list of available remote playback devices to the value of newDevices.
Stop observing remote playback devices availability

When a cancelWatchAvailability() method is called, the user agent MUST run the following steps:

Input
id, the callback identifier.
Output
promise, a promise.
  1. Let promise be a new promise.
  2. Return promise, and run the following steps below:
  3. If the disableRemotePlayback attribute is present for the media element, reject promise with InvalidStateError and abort all the remaining steps.
  4. If the parameter id is undefined, clear the set of availability callbacks.
  5. Otherwise, if id matches the callbackId for any entry in the set of availability callbacks, remove the entry from the set.
  6. Otherwise, reject promise with NotFoundError and abort all the remaining steps.
  7. If the set of availability callbacks is now empty and there is no pending request to initiate remote playback, cancel any pending task to monitor the list of available remote playback devices for power saving purposes.
  8. Fulfill promise.
The mechanism used to monitor remote playback devices availability and determine the compatibility of a remote playback device with the selected availability sources set is left to the user agent.

Prompt user for changing remote playback state

When the prompt() method is called, the user agent MUST run the following steps:

Input
None, but the algorithm references the media element, its remote property and its availability sources set.
Output
A promise.
  1. Let promise be a new promise.
  2. Return promise and continue running these steps in parallel.
  3. If the disableRemotePlayback attribute is present for the media element, reject the promise with InvalidStateError and abort all the remaining steps.
  4. If there is already an unsettled promise from a previous call to prompt for the same media element or even for the same browsing context, the user agent MAY reject promise with an OperationError exception and abort all remaining steps.
    The rationale here is that the user agent might show a dialog that's modal to either the media element or the browsing context. In such a case, the second call to prompt() would not be able to show any UI.
  5. If the algorithm isn't allowed to show a popup, reject promise with an InvalidAccessError exception and abort these steps.
  6. OPTIONALLY, if the user agent knows a priori that remote playback of this particular media element is not feasible (independent of the current state or the list of available remote playback devices), reject promise with a NotSupportedError and abort all remaining steps.
    An example of this situation is when the user agent only supports media flinging, and the media element's source is not a URL that can be passed to a remote playback device.
  7. If the user agent needs to show the list of available remote playback devices and is not monitoring the list of available remote playback devices, run the steps to monitor the list of available remote playback devices in parallel.
  8. If remote playback is unavailable and will remain so before the request for user permission is complete, reject promise with a NotFoundError exception and abort all remaining steps.
  9. Request user permission to change remote playback state.
    An example UI to request permission would allow the user to pick a new remote playback device, switch between local or remote playback devices, or disconnect from a remote playback device.
    The user may have already selected a remote playback device for a related purpose, for example, to mirror the contents of the display or the current page. In that case, the user agent may choose to skip the UI to select a device and proceed immediately to the next step.
  10. If the user picked a remote playback device device to initiate remote playback with, the user agent MUST run the following steps:
    1. Set the state of the remote object to connecting.
    2. Fulfill promise.
    3. Queue a task to fire a simple event with the name connecting at the remote property of the media element. The event must not bubble, must not be cancelable, and has no default action.
    4. Establish a connection with the remote playback device device for the media element.

    By picking a remote playback device the user grants permission to use the device.

  11. Otherwise, if the user chose to disconnect from the remote playback device device, the user agent MUST run the following steps:
    1. Fulfill promise.
    2. Run the disconnect from a remote playback device algorithm for the device.
  12. Otherwise, the user is considered to deny permission to use the device, so reject promise with NotAllowedError exception and hide the UI shown by the user agent.
The details of implementing the UI and device selection are left to the user agent; for example it can show the user a dialog and allow the user to select an available device (granting permission), or cancel the selection (denying permission). In most cases, available devices will advertise a user friendly name along with locale and text direction information for that name. The user agent is encouraged to render this user friendly name, using its locale and text direction when they are known.
The algorithm to select the remote playback source for a selected device depends on the user agent and supported remote playback device types. For example, in case of media mirroring the user agent can simply follow the HTMLMediaElement's resource selection algorithm. However, if media remoting or media flinging is used, the best media source can depend on the selected remote playback device fetch and playback capabilities.

The state attribute

The state attribute represents the RemotePlayback connection's current state. It can take one of the values of RemotePlaybackState depending on the connection state:

  • connecting means that the user agent is attempting to initiate remote playback with the selected remote playback device. This is the initial state when the promise returned by prompt() is fulfilled. The local playback of the media element continues in this state and media commands still take effect on the local playback state.
  • connected means that the transition from local to remote playback has finished and all media commands now take effect on the remote playback state.
  • disconnected means that the remote playback has not been initiated, has failed to initiate or has been stopped. All media commands will take effect on the local playback state. The remote playback can be initiated through a call to prompt().

Establishing a connection with a remote playback device

When the user agent is to establish a connection with the remote playback device, it MUST run the following steps:

Input
remote, the RemotePlayback object that is to be connected.
device, the remote playback device to connect to.
  1. If the state of remote is not equal to connecting, abort all the remaining steps.
  2. Request connection of remote to device. The implementation of this step is specific to the user agent.
  3. If connection completes successfully, queue a task to run the following steps:
    1. Set the state of remote to connected.
    2. Fire a simple event named connect at remote.
    3. Synchronize the current media element state with the remote playback state. The implementation of this step is specific to the user agent.
  4. If connection fails, queue a task to run the following steps:
    1. Set the remote playback state of remote to disconnected.
    2. Fire a simple event named disconnect at remote.

The user agent SHOULD pause local audio and video output of the media element while the remote playback state is connected.

If the user agent is exposing a user interface to the user for the media element (i.e., using default controls), the user agent SHOULD convey the fact that the remote playback state is connected through an icon or other means.

The mechanism that is used to connect the user agent with the remote playback device and play the remote playback source is an implementation choice of the user agent. The connection will likely have to provide a two-way messaging abstraction capable of carrying media commands to the remote playback device and receiving media playback state in order to keep the media element state and remote playback state in sync (unless media mirroring is used).
The user agent is encouraged to pass locale and text direction information to the remote playback device when possible, so that the remote playback device can adjust its user interface and operational functions to locale-specific attributes that reflect the user's preferences. For example, the remote playback device can use that information to choose the same default text track as the user agent, as well as to set the HTTP Accept-Language header it sends to fetch media resources.
The user agent should not render output from the media element while it is in a connected state and its content is being rendered on a remote playback device.

Browser initiated remote playback

A user agent MAY support connecting to a remote playback device from the browser. This can be done by adding appropriate media controls to the user interface that is exposed to the user, or when the user activates system-wide mirroring of the display. This feature is known as browser initiated remote playback. A user agent that supports browser initiated remote playback SHOULD initiate the remote playback only when the user has expressed an intention to do so via a user gesture, for example by clicking a button in the browser.

If the user agent supports browser initiated remote playback, the state attribute MUST reflect the current state of the connection to the remote playback device. When the browser initiates or terminates remote playback, it MUST fire the corresponding events by following the algorithms to establish a connection with the remote playback device and disconnect from a remote playback device.

If the browser will initiate remote playback on a newly created media element, it SHOULD initialize the value of its state attribute to connecting and then follow the steps to establish a connection with the remote playback device.

Media commands and media playback state

The HTMLMediaElement interface interacts with the remotely played media as soon as the connection with the remote playback device is established.

When the state of a RemotePlayback object is connected, the following conditions relate the local playback state, the media element state, and the remote playback state:

The user agent MUST send all media commands issued on the associated HTMLMediaElement object to the remote playback device in order to change its remote playback state;
The remote playback device SHOULD implement all media commands sent by the user agent;
The remote playback device SHOULD send updates of the remote playback state to the user agent that affect any attribute exposed through the media element state;
The user agent MUST process all updates of the remote playback state received from the remote playback device and update the local playback state of the media element accordingly.

If sending any command fails, the user agent MAY disconnect from a remote playback device.

The remote playback device can implement a subset of the capabilities of the playback engine of the user agent, and some HTMLMediaElement APIs do not always make sense to use during remote playback. In this case, the local playback state is expected to reflect as closely as possible the actual remote playback state after a media command that is not supported during remote playback.

For example, after calling fastSeek() while connected to a remote playback device that does not support it, the seeking attribute of the HTMLMediaElement is expected to remain false and no seeking event is to be fired.

Disconnecting from a remote playback device

When the user agent is to disconnect from a remote playback device, it MUST do the following:

Input
remote, the RemotePlayback object representing the playback to be stopped.
device, the remote playback device to disconnect from.
  1. If the state of remote is disconnected, abort all remaining steps.
  2. Queue a task to run the following steps:
    1. Request disconnection of remote from the device. The implementation of this step is specific to the user agent.
    2. Change the remote's state to disconnected.
    3. Fire an event with the name disconnect at remote.
    4. Synchronize the current media element state with the local playback state. The implementation of this step is specific to the user agent.

If the remote playback device is abruptly disconnected during playback (for example, by power loss or a network disconnection), the user agent SHOULD run the steps to monitor the list of available remote playback devices before the steps to disconnect from a remote playback device. This allows callbacks in the set of availability callbacks to be invoked before the disconnect event is fired, so the page can update itself to show resumption of playback is not possible.

The remote playback device might not actually stop playback of the media when requested by the user agent; it depends on the implementation of the user agent and the remote playback device. In this case, stopping remote playback means that the user agent merely disconnects from the remote playback device and the media element switches to the disconnected state.

Event Handlers

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the RemotePlayback interface:

Event handler Event handler event type
onconnecting connecting
onconnect connect
ondisconnect disconnect

Extensions to HTMLMediaElement 

          partial interface HTMLMediaElement {
            [SameObject] readonly attribute RemotePlayback remote;

            [CEReactions] attribute boolean disableRemotePlayback;
          };

The remote attribute MUST return the RemotePlayback instance associated with the media element.

The disableRemotePlayback attribute

Some pages may wish to disable remote playback of a media element; for example, they may prefer to use a PresentationRequest to present a complete document on a presentation screen. To support this use case, a new disableRemotePlayback attribute is added to the list of content attributes for audio and video elements.

A corresponding disableRemotePlayback IDL attribute which reflects the value of each element’s disableRemotePlayback content attribute is added to the HTMLMediaElement interface. The disableRemotePlayback IDL attribute MUST reflect the content attribute of the same name.

Disabling remote playback

If the disableRemotePlayback attribute is present on the media element, the user agent MUST NOT play the media element remotely or present any UI to do so.

When the disableRemotePlayback attribute is added to the media element, the user agent MUST run the steps to disable remote playback:

  1. Reject any pending promises returned by the RemotePlayback methods with InvalidStateError.
  2. Clear the set of availability callbacks for the media element.
  3. If its state is not disconnected, run the disconnect from a remote playback device algorithm for the remote playback device the media element is connected or connecting to.

Security and privacy considerations

Personally identifiable information

Firing the callback provided via the watchAvailability() method reveals one bit of information about the presence (or non-presence) of a remote playback device typically discovered through the local area network. This could be used in conjunction with other information for fingerprinting the user. However, this information is also dependent on the user's local network context, so the risk is minimized. Also, by design, the human readable name of a remote playback device is not revealed to the page.

The API enables monitoring the list of available remote playback devices. How the user agent determines the compatibility and availability of a remote playback device with a media element's resource is an implementation detail. If a user agent matches a media resource to a particular type of device to determine its availability, this feature can be used to probe information about which remote playback device the user has without user consent.

The user agent should not monitor the list of available remote playback devices if the user disables background monitoring through a browser setting.

User interface guidelines

Origin display

When the user is asked permission to use a remote playback device during the steps to change remote playback state, the user agent should make it clear what origin the request is coming from.

Display of the origin requesting remote playback will help the user understand what content is making the request, especially when the request is initiated from a nested browsing context. For example, embedded content may try to convince the user to click to trigger a request to start an unwanted remote playback.

Showing the origin that will be presented will help the user know if that content is from a potentially trustworthy origin (e.g., https:), and corresponds to a known or expected site.

Device Access

The Remote Playback API abstracts away what "local" means for displays, meaning that it exposes network-accessible displays as though they were local displays. The Remote Playback API requires user permission for a page to access any display to mitigate issues that could arise, such as showing unwanted content on a display viewable by others.

Messaging between the local and remote playback devices

This specification will not mandate communication protocols between the local playback device and the remote playback device, but the user agent should set some guarantees of message confidentiality and authenticity between them.

Secure Contexts

The Remote Playback API is not limited to [[SECURE-CONTEXTS]] because it exposes a feature to web applications that user agents usually offer natively on all media regardless of the browsing context. A user agent can limit the API to [[SECURE-CONTEXTS]] by always returning an empty list as part of the monitor the list of available remote playback devices algorithm in contexts that are not secure.

Candidate Recommendation exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. Additionally, implementations must demonstrate support for the media remoting and media flinging cases, either within the same product or within different products.

For the purposes of these criteria, we define the following terms:

Independent
Each implementation must be developed by a different party, and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
Interoperable
Passing the respective test case(s) in the official test suite.
Implementation
A user agent which:
  1. implements the specification.
  2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or "nightly build"). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
  3. is not experimental (i.e. a version specifically designed to pass the test suite and not intended for normal usage going forward).