publishing/media_overlay.html

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    <title>Technologies for Media Overlay</title>
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        <h1>Technologies for Media Overlay</h1>
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           Media Overlay is essential for many types of (Web) Publications (e.g., in accessibility or in children’s books). EPUB relies on a subset of SMIL to achieve that; this is not available for Web Publications, because browsers do not implement SMIL. An alternative is to define a simple set of features for a minimal level of declarative media synchronization, and use that as a basis for Media Overlay on the Web.
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            <h2>Specifications in progress</h2>
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                <li data-feature="Web Animation API">Beyond text tracks, Web pages may contain many other time-based animations with which synchronization can be useful; the <a data-featureid="web-animations">Web Animations API</a> offers the tools needed to set up these synchronization points. This may become the basis for the Media Overlay work both in terms of model and terminology, as well as possible implementation work.</li>

                <li data-feature="HTML Extensions">Media Overlay <em>may</em> require some simple extensions to HTML (e.g., additional attributes for Media Overlays). The <a data-featureid="html-extensions">HTML Extension Specifications</a> provides a good starting point for the roadmap to such extensions.</li>

                <li data-feature="Timed Text Markup Language"><a data-featureid="ttml">TTML2</a> Content may be used directly as a distribution format, for example, providing a standard content format to reference from a <code>track</code> element in an HTML document, or a <code>text</code> or <code>textstream</code> media element in a SMIL&nbsp;3.0 document.</li>

                <li data-feature="TTML Profiles for Internet Media Subtitles and Captions">
                    <a data-featureid="ttml-imsc11">TTML Profiles for Internet Media Subtitles and Captions</a> defines two profiles of <a href="https://www.w3.org/TR/ttml2/">TTML2</a>: a text-only profile and an image-only profile. These profiles are intended to be used across subtitle and caption delivery applications worldwide, thereby simplifying interoperability, consistent rendering and conversion to other subtitling and captioning formats.
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                <li data-feature="WebVTT: The Web Video Text Tracks Format">The main use of <a data-featureid="webvtt">WebVTT</a> is for marking up external text track resources in connection with the HTML <code>track</code> element. WebVTT files provide captions or subtitles for video content, and also text video descriptions used for accessibility purposes&nbsp;[<a href="http://www.w3.org/TR/media-accessibility-reqs/">MAUR</a>], chapters for content navigation, and more generally any form of metadata that is time-aligned with audio or video content.</li>
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            <h2>Exploratory work</h2>
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                <li data-feature="Multi-device synchronization">There are a number of cases that need synchronization of several tracks in the same page, for instance to synchronize the sign-language transcript of an audio track with its associated video. The <code>MediaController</code> interface, initially defined in HTML5, was dropped from the HTML specification due to very limited implementation support. The <a data-featureid="timing">Timing Object</a> specification proposes a mechanism to bring shared on-line locks to browsers and ease synchronization of heterogeneous content within a single device and across devices. This may be useful for the Media Overlay work.</li>
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