This document collects use cases and requirements for improved support for timed events related to audio or video media on the web, where synchronization to a playing audio or video media stream is needed, and makes recommendations for new or changed web APIs to realize these requirements. The goal is to extend the existing support in HTML for text track cue events to add support for dynamic content replacement cues and generic metadata events that drive synchronized interactive media experiences, and improve the timing accuracy of rendering of web content intended to be synchronized with audio or video media playback.

The Media & Entertainment Interest Group may update these use cases and requirements over time. Development of new web APIs based on the requirements described here, for example, DataCue, will proceed in the Web Platform Incubator Community Group (WICG), with the goal of eventual standardization within a W3C Working Group. Contributors to this document are encouraged to participate in the WICG. Where the requirements described here affect the HTML specification, contributors will follow up with WHATWG. The Interest Group will continue to track these developments and provide input and review feedback on how any proposed API meets these requirements.


There is a need in the media industry for an API to support metadata events synchronized to audio or video media, specifically for both out-of-band event streams and in-band discrete events (for example, MPD and emsg events in MPEG-DASH). These media timed events can be used to support use cases such as dynamic content replacement, ad insertion, or presentation of supplemental content alongside the audio or video, or more generally, making changes to a web page, or executing application code triggered from JavaScript events, at specific points on the media timeline of an audio or video media stream.


The following terms are used in this document:

The following terms are defined in [[HTML]]:

The following term is defined in [[HR-TIME]]:

The following term is defined in [[WEBVTT]]:

Use cases

Media timed events carry metadata that is related to points in time, or regions of time on the media timeline, which can be used to trigger retrieval and/or rendering of web resources synchronized with media playback. Such resources can be used to enhance user experience in the context of media that is being rendered. Some examples include display of social media feeds corresponding to a live video stream such as a sporting event, banner advertisements for sponsored content, accessibility-related assets such as large print rendering of captions, and display of track titles or images alongside an audio stream.

The following sections describe a few use cases in more detail.

Dynamic content insertion

A media content provider wants to allow insertion of content, such as personalised video, local news, or advertisements, into a video media stream that contains the main program content. To achieve this, media timed events can be used to describe the points on the media timeline, known as splice points, where switching playback to inserted content is possible.

The Society for Cable and Televison Engineers (SCTE) specification "Digital Program Insertion Cueing for Cable" [[SCTE35]] defines a data cue format for describing such insertion points. Use of these cues in MPEG-DASH and HLS streams is described in [[SCTE35]], sections 12.1 and 12.2.

Audio stream with titles and images

A media content provider wants to provide visual information alongside an audio stream, such as an image of the artist and title of the current playing track, to give users live information about the content they are listening to.

HLS timed metadata [[HLS-TIMED-METADATA]] uses in-band ID3 metadata to carry the artist and title information, and image content. RadioVIS in DVB ([[DVB-DASH]], section 9.1.7) defines in-band event messages that contain image URLs and text messages to be displayed, with information about when the content should be displayed in relation to the media timeline.

Control messages for media streaming clients

A media streaming server uses media timed events to send control messages to media client library, such as dash.js. Typically segmented streaming protocols such as HLS and MPEG-DASH make use of a manifest document that informs the client of the available encodings of a media stream, e.g., the Media Presentation Description (MPD) document in [[MPEGDASH]].

Should any of the content in the manifest document need to change, the client should refresh it by requesting an updated copy from the server. Section 5.10.4 of [[MPEGDASH]] describes an MPEG-DASH specific event that is used to notify a client application. An in-band emsg event is used as an alternative to setting a cache duration in the response to the HTTP request for the manifest, so the client can refresh the MPD when it actually changes, as opposed to waiting for a cache duration expiry period to elapse. This also has the benefit of reducing the load on HTTP servers caused by frequent server requests.

Reference: M&E IG call 1 Feb 2018: Minutes, [[DASH-EVENTING]].

Subtitle and caption rendering synchronization

A subtitle or caption author wants ensure that subtitle changes are aligned as closely as possible to shot changes in the video. The BBC Subtitle Guidelines [[BBC-SUBTITLE]] describes authoring best practices. In particular, in section 6.1 authors are advised "it is likely to be less tiring for the viewer if shot changes and subtitle changes occur at the same time. Many subtitles therefore start on the first frame of the shot and end on the last frame."

Synchronized map animations

A user records footage with metadata, including geolocation, on a mobile video device, e.g., drone or dashcam, to share on the web alongside a map, e.g., OpenStreetMap.

[[WEBVMT]] is an open format for metadata cues, synchronized with a timed media file, that can be used to drive an online map rendered in a separate HTML element alongside the media element on the web page. The media playhead position controls presentation and animation of the map, e.g., pan and zoom, and allows annotations to be added and removed, e.g., markers, at specified times during media playback. Control can also be overridden by the user with the usual interactive features of the map at any time, e.g., zoom. Concrete examples are provided by the tech demos at the WebVMT website.

Media analysis visualization

A video image analysis system processes a media stream to detect and recognize objects shown in the video. This system generates metadata describing the objects, including timestamps that describe the when the objects are visible, together with position information (e.g., bounding boxes). A web application then uses this timed metadata to overlay labels and annotations on the video using HTML and CSS.

Presentation of auxiliary content in live media

During a live media presentation, dynamic and unpredictable events may occur which cause temporary suspension of the media presentation. During that suspension interval, auxiliary content such as the presentation of UI controls and media files, may be unavailable. Depending on the specific user engagement (or not) with the UI controls and the time at which any such engagement occurs, specific web resources may be rendered at defined times in a synchronized manner. For example, a multimedia A/V clip along with subtitles corresponding to an advertisement, and which were previously downloaded and cached by the UA, are played out.

Related industry specifications

This section describes existing media industry specifications and standards that specify carriage of media timed events, or otherwise provide requirements for web APIs related to the triggering of media timed events.

MPEG Common Media Application Format (CMAF)

MPEG Common Media Application Format (CMAF) [[MPEGCMAF]] is a media container format optimized for large scale delivery of a single encrypted, adaptable multimedia presentation to a wide range of devices and adaptive streaming methods, including HTTP Live Streaming [[RFC8216]] and MPEG-DASH [[MPEGDASH]]. It is based on the ISO BMFF [[ISOBMFF]] and supports the AVC, AAC, HEVC codecs, Common Encryption (CENC), and subtitles using IMSC1 and WebVTT. The goal is to reduce media storage and delivery costs by using a single common media format across different client devices.

CMAF media may contain in-band events in the form of Event Message (emsg) boxes in ISO BMFF files. emsg is specified in [[MPEGDASH]], section, and described in more detail in the following section of this document.


MPEG-DASH is an adaptive bitrate streaming technique in which the audio and video media is partitioned into segments. The Media Presentation Description (MPD) is an XML document that contains metadata required by a DASH client to access the media segments and to provide the streaming service to the user. The media segments can use any codec, typically within a fragmented MP4 (ISO BMFF) container or MPEG-2 transport stream.

In MPEG-DASH, media timed events may be delivered either in-band or out-of-band:

An emsg event contains the following information, as specified in [[MPEGDASH]], section

HTTP Live Streaming

HTTP Live Streaming (HLS) allows for delivery of timed metadata events, both in-band and out-of-band:

An EXT-X-DATERANGE tag contains the following information, as specified in [[RFC8216]], section


HbbTV is an interactive TV application standard that supports both broadcast (DVB) media delivery, and internet streaming using MPEG-DASH. The HbbTV application environment is based on HTML and JavaScript. MPEG-DASH streaming is implemented nativey by the user agent, rather than through a JavaScript web application using Media Source Extensions.

HbbTV includes support for emsg events ([[DVB-DASH]], section 9.1) and requires this be mapped to HTML5 DataCue ([[HBBTV]], section 9.3.2). The revision of HTML5 referenced by [[HBBTV]] is [[html51-20151008]]. This feature is included in user agents shipping in connected TVs across Europe from 2017.

The HbbTV device test suite includes test pages and streams that cover emsg support. HbbTV has a reference application and content for DASH+DRM which includes emsg support.

DASH Industry Forum APIs for Interactivity

The DASH-IF InterOp Working Group has an ongoing work item, DAInty, "DASH APIs for Interactivity", which aims to specify a set of APIs between the DASH client/player and interactivity-capable applications, for both web and native applications [[DASHIFIOP]]. The origin of this work is a related 3GPP work item on Service Interactivity [[3GPP-INTERACTIVITY]]. The objective is to provide service enablers for user engagement with auxiliary content and UIs on mobile device during live or time-shifted viewing of streaming content delivered over 3GPP broadcast or unicast bearers, and the measurement and reporting of such interactive consumption.

Two APIs are being developed that are relevant to the scope of the present document:

Two modes for dispatching events are defined [[DASHIF-EVENTS]]. In on-receive mode, events are dispatched at the time the event arrives, and in on-start mode, events are dispatched at the given time on the media timeline. The "arrival" of events from the DASH client perspective may be either static or pre-provisioned, in the case MPD Events, or dynamic in the case of in-band events carried in emsg boxes. The application can register with the DASH client which mode to use.


The Society for Cable and Televison Engineers (SCTE) has produced the SCTE-35 specification "Digital Program Insertion Cueing for Cable" [[SCTE35]], which defines a data cue format for describing insertion points, to support the dynamic content insertion use case.

[[SCTE214-1]] section 6.7 describes the carriage of SCTE-35 events in a MPEG-DASH MPD document, as out-of-band events. [[SCTE214-2]] section 9 and [[SCTE214-3]] section 7.3 describe the carriage of SCTE-35 events as in-band events in MPEG-DASH using MPEG2-TS and ISO BMFF respectively, using emsg.

[[RFC8216]] section specifies how to map SCTE-35 events into HLS timed metadata, using the EXT-X-DATERANGE tag, with SCTE35-CMD, SCTE35-OUT, and SCTE35-IN attributes.

[[SCTE35]] section 9.1 describes the requirements for content splicing: "In order to give advance warning of the impending splice (a pre-roll function), the splice_insert() command could be sent multiple times before the splice point. For example, the splice_insert() command could be sent at 8, 5, 4 and 2 seconds prior to the packet containing the related splice point. In order to meet other splicing deadlines in the system, any message received with less than 4 seconds of advance notice may not create the desired result."

This places an implicit requirement on the user agent in handling of media-timed events related to insertion cues. The content originator may provide the cue in advance with as little as 2 seconds of the insertion time. Therefore the propagation of the event data associated with the insertion cue to the application by the user agent should be considerably less than 2 seconds.

MPEG Working Draft on Carriage of Web Resources in ISO BMFF

The MPEG Working Draft on Carriage of Web Resources in ISO BMFF [[WEB-ISOBMFF]] is a draft document that specifies the use of the ISO BMFF container format for the storage and delivery of web content. The goal is to allow web resources (HTML, JavaScript, etc.) to be parsed from the storage and processed by a user agent at specific presentation times on the media timeline, and so be synchronized with other tracks within the container, such as audio, video, and subtitles.

The Media & Entertainment Interest Group is actively tracking this work is open to discussing specific requirements for media timed events as development progresses.


[[WEBVTT]] is a W3C specification that provides a format for web video text tracks. A VTTCue is a text track cue, and may have attributes that affect rendering of the cue text on a web page. WebVTT metadata cues are text that is aligned to the media timeline. Web applications can use VTTCue to schedule out-of-band metadata events by serializing the event data to a string format (JSON, for example) when creating the cue, and deserializing the data when the cue is triggered.

Web applications can also use VTTCue to trigger rendering of out-of-band delivered timed text cues, such as TTML or IMSC format captions.

Gap analysis

This section describes gaps in existing existing web platform capabilities needed to support the use cases and requirements described in this document. Where applicable, this section also describes how existing web platform features can be used as workarounds, and any associated limitations.

MPEG-DASH and ISO BMFF emsg events

The DataCue API has been previously discussed as a means to deliver in-band event data to web applications, but this is not implemented in all of the main browser engines. It is included in the 18 October 2018 HTML 5.3 draft [[HTML53-20181018]], but is not included in [[HTML]]. See discussion here and notes on implementation status here.

WebKit supports a DataCue interface that extends HTML5 DataCue with two attributes to support non-text metadata, type and value.

          interface DataCue : TextTrackCue {
            attribute ArrayBuffer data; // Always empty

            // Proposed extensions.
            attribute any value;
            readonly attribute DOMString type;

type is a string identifying the type of metadata:

WebKit DataCue metadata types
"" QuickTime User Data
"" QuickTime Metadata
"" iTunes metadata
"org.mp4ra" MPEG-4 metadata
"org.id3" ID3 metadata

and value is an object with the metadata item key, data, and optionally a locale:

          value = {
            key: String
            data: String | Number | Array | ArrayBuffer | Object
            locale: String

Neither [[MSE-BYTE-STREAM-FORMAT-ISOBMFF]] nor [[INBANDTRACKS]] describe handling of emsg boxes.

On resource constrained devices such as smart TVs and streaming sticks, parsing media segments to extract event information leads to a significant performance penalty, which can have an impact on UI rendering updates if this is done on the UI thread. There can also be an impact on the battery life of mobile devices. Given that the media segments will be parsed anyway by the user agent, parsing in JavaScript is an expensive overhead that could be avoided.

Avoiding parsing in JavaScript is also important for low latency video streaming applications, where minimizing the time taken to pass media content through to the media element's playback buffer is essential.

[[HBBTV]] section 9.3.2 describes a mapping between the emsg fields described above and the TextTrack and DataCue APIs. A TextTrack instance is created for each event stream signalled in the MPD document (as identified by the schemeIdUri and value), and the inBandMetadataTrackDispatchType TextTrack attribute contains the scheme_id_uri and value values. Because HbbTV devices include a native DASH client, parsing of the MPD document and creation of the TextTracks is done by the user agent, rather than by application JavaScript code.

Synchronized rendering of web resources

In browsers, non media web rendering is handled through repaint operations at a rate that generally matches the display refresh rate (e.g., 60 times per second), following the user's wall clock. A web application can schedule actions and render web content at specific points on the user's wall clock, notably through, setTimeout(), setInterval(), and requestAnimationFrame().

In most cases, media rendering follows a different path, be it because it gets handled by a dedicated background process or by dedicated hardware circuitry. As a result, progress along the media timeline may follow a clock different from the user's wall clock. [[HTML]] recommends that the media clock approximate the user's wall clock but does not require it to match the user's wall clock.

To synchronize rendering of web content to a video with frame accuracy, a web application needs:

The following sub-sections discusses mechanisms currently available to web applications to track progress on the media timeline and render content at frame boundaries.

Using cues to track progress on the media timeline

Cues (e.g., TextTrackCue, or VTTCue) are units of time-sensitive data on a media timeline [[HTML]]. The time marches on steps in [[HTML]] control the firing of cue events during media playback. Time marches on requires a timeupdate event to be fired at the media element between 15 and 250 milliseconds since the last such event, and this requirement therefore specifies the rate at which time marches on is executed during playback. In practice it has been found that the timing varies between browser implementations.

There are two methods a web application can use to handle cues:

  • Add an oncuechange handler function to the TextTrack and inspect the track's activeCues list. Because activeCues contains the list of cues that are active at the time that time marches on is run, it is possible for cues to be missed by a web application using this method, where cues appear on the media timeline between successive executions of time marches on during media playback. This may occur if the cues have short duration, or by a long-running event handler function.
  • Add onenter and onexit handler functions to each cue. The time marches on steps guarantee that enter and exit events will be fired for all cues, including those that appear on the media timeline between successive executions of time marches on during media playback. The timing accuracy of these events varies between browser implementations, as the firing of the events is controlled by the rate of execution of time marches on.

An issue with handling of text track and data cue events in HbbTV was reported in 2013. HbbTV requires the user agent to implement an MPEG-DASH client, and so applications must use the first of the above methods for cue handling, which means that applications can miss cues as described above. A similar issue has been filed against the HTML specification.

Using timeupdate events from the media element

Another approach to synchronizing rendering of web content to media playback is to use the timeupdate event, and for the web application to manage the media timed events to be triggered, rather than use the text track cue APIs in [[HTML]]. This approach has the same synchronization limitations as described above due to the 250 millisecond update rate specified in time marches on, and so is explicitly discouraged in [[HTML]]. In addition, the timing variability of timeupdate events between browser engines makes them unreliable for the purpose of synchronized rendering of web content.

Polling the current position on the media timeline

Synchronization accuracy can be improved by polling the media element's currentTime property from a setInterval() callback, or by using requestAnimationFrame() for greater accuracy. This technique can be useful in where content should be animated smoothly in synchronicity with the media, for example, rendering a playhead position marker in an audio waveform visualization, or displaying web content at specific points on the media timeline. However, the use of setInterval() or requestAnimationFrame() for media synchronized rendering is CPU intensive.

Detecting when the next media frame will be rendered

[[HTML]] does not expose any precise mechanism to assess the time, from a user's wall clock perspective, at which a particular media frame is going to be rendered. A web application may only infer this information by looking at the media element's currentTime property to infer the frame being rendered and the time at which the user will see the next frame. This has several limitations:

  • currentTime is represented as a double value, which does not allow to identify individual frames due to rounding errors. This is a known issue.
  • currentTime is updated at a user-agent defined rate (typically the rate at which time marches on runs), and is kept stable while scripts are running. When a web application reads currentTime, it cannot tell when this property was last updated, and thus cannot reliably assess whether this property still represents the frame currently being rendered.


This section describes recommendations from the Media & Entertainment Interest Group for the development of a generic media timed event API, and associated synchronization considerations.

Subscribing to event streams

The API should allow web applications to subscribe to receive specific event streams by event type. For example, to support MPEG-DASH emsg and MPD events, the API should allow subscription by id and (optional) value. This is to make receiving events opt-in from the application point of view. The user agent should deliver only those events to a web application for which the application has subscribed. The API should also allow web applications to unsubscribe from specific event streams by event type.

Out-of-band events

To be able to handle out of band events, including MPEG-DASH MPD events, the API should allow web applications to create events to be added to the media timeline, to be triggered by the user agent. The API should allow the web application to provide all necessary parameters to define the event, including start and end times, event type, and data payload. The payload should be any data type (e.g., the set of types supported by the WebKit DataCue). For MPEG-DASH MPD events, the event type is defined by the id and (optional) value fields.

Event triggering

For those events that the application has subscribed to receive, the API should:

The API should provide guarantees that no events can be missed during linear playback of the media.

In-band event processing

We recommend updating [[INBANDTRACKS]] to describe handling of in-band media timed events supported on the web platform, following a registry approach with one specification per media format that describes the event details for that format.

MPEG-DASH events

We recommend that browser engines support MPEG-DASH emsg in-band events and MPD out-of-band events, as part of their support for the MPEG Common Media Application Format (CMAF) [[MPEGCMAF]].


In order to achieve greater synchronization accuracy between media playback and web content rendered by an application, the time marches on steps in [[HTML]] should be modified to allow delivery of media timed event start time and end time notifications within 20 milliseconds of their positions on the media timeline.

Additionally, to allow such synchronization to happen at frame boundaries, we recommend introducing a mechanism that would allow a web application to accurately predict, using the user's wall clock, when the next frame will be rendered (e.g., as done in the Web Audio API).


Thanks to François Daoust, Charles Lo, Nigel Megitt, Jon Piesing, Rob Smith, and Mark Vickers for their contributions and feedback on this document.