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JavaScript library for rendering IMSC Text and Image Profile documents to HTML5


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imscJS is a JavaScript library for rendering IMSC 1.0.1 and IMSC 1.1 documents to HTML5. IMSC is a profile of TTML 2 designed for subtitle and caption delivery worldwide.

A sample web app that uses imscJS is available at

Documentation is available on MDN.


imscJS is primarily developed on Firefox. Latest versions of Chrome, Safari, and Microsoft Edge are intended to be supported nevertheless, albeit with potentially reduced capabilities. In particular, advanced ruby layout is currently only supported by Firefox.

imscJS is intended to reflect the most recent published versions of IMSC 1.0.1 and IMSC 1.1. These publications are routinely clarified by proposed resolutions to issues captured in their respective bug trackers.

imscJS bugs are tracked at


(required) sax-js 1.2.1

Rendering to HTML5 requires a browser environment, but parsing an IMSC document and transforming it into ISDs does not.


(required) node.js (see package.json for a complete list of dependencies)

(recommended) git


  • run the build target defined in Gruntfile.js using grunt.

  • the resulting imsc.js and sax.js files at build/public_html/libs are, respectively, the imscJS library and its sax-js dependency. For example, both libraries can be included in a web page as follows:

    <script src="libs/sax.js"></script>
    <script src="libs/imsc.js"></script>

See BUILD ARTIFACTS for a full list of build artifacts, and TESTS AND SAMPLES for a list of samples and tests available.



imscJS renders an IMSC document in three distinct steps:

  • fromXML(xmlstring, errorHandler, metadataHandler) parses the document and returns a TT object. The latter contains opaque representation of the document and exposes the method getMediaTimeEvents() that returns a list of time offsets (in seconds) of the ISD, i.e. the points in time where the visual representation of the document change.

  • generateISD(tt, offset, errorHandler) creates a canonical representation of the document (provided as a TT object generated by fromXML()) at a point in time (offset parameter). This point in time does not have to be one of the values returned by getMediaTimeEvents(). For example, given an ISOBMFF sample covering the interval [a, b[, generateISD(tt, offset, errorHandler) would be called first with offset = a, then in turn with offset set to each value of getMediaTimeEvents() that fall in the interval ]a, b[.

  • renderHTML(isd, element, imgResolver, eheight, ewidth, displayForcedOnlyMode, errorHandler, previousISDState, enableRollUp) renders an isd object returned by generateISD() into a newly-created div element that is appended to the element. The element must be attached to the DOM. The height and width of the child div element are equal to eheight and ewidth if not null, or clientWidth and clientHeight of the parent element otherwise. Images URIs specified in smpte:background attributes are mapped to image resource URLs by the imgResolver function. The latter takes the value of the smpte:background attribute URI and an img DOM element as input and is expected to set the src attribute of the img DOM element to the absolute URI of the image. displayForcedOnlyMode sets the (boolean) value of the IMSC displayForcedOnlyMode parameter. enableRollUp enables roll-up as specified in CEA 708. previousISDState maintains states across calls, e.g. for roll-up processing.

In each step, the caller can provide an errorHandler to be notified of events during processing. The errorHandler may define four methods: info, warn, error and fatal. Each is called with a string argument describing the event, and will cause processing to terminate if it returns true.

Inline documentation provides additional information.


imscJS consists of the following modules, which can be used in a node environment using the require functionality, or standalone, in which case each module hosts its definitions under a global name (the token between parantheses):

  • doc.js (imscDoc): parses an IMSC document into an in-memory TT object
  • isd.js (imscISD): generates an ISD object from a TT object
  • html.js (imscHTML): generates an HTML fragment from an ISD object
  • names.js (imscNames): common constants
  • styles.js (imscStyles): defines TTML styling attributes processing
  • utils.js (imscUtils): common utility functions


imscJS is built using the build:release or build:debug Grunt tasks -- the build task is an alias of build:debug.

The dist directory contains the following build artifacts:

  • imsc.all.debug.js: Non-minified build that includes the sax-js dependency.
  • imsc.all.min.js: Minified build that includes the sax-js dependency.
  • imsc.debug.js: Non-minified build that does not include the sax-js dependency.
  • imsc.min.js: Minified build that does not include the sax-js dependency.

The build/public_html/libs/imsc.js files is identical to:

  • imsc.debug.js, if the build:debug task is executed.
  • imsc.min.js, if the build:release task is executed.


imscJS is released as an NPM package under imsc. The dev distribution tag indicates pre-releases.

Builds/dist are available on the unpkg CDN under the dist directory.

To access the latest builds, please consult the release page.


W3C Test Suite

imscJS imports the IMSC test suite as a submodule at src/test/resources/imsc-tests. The gen-renders.html web app can be used to generate PNG renderings as as well intermediary files from these tests.

Unit tests

Unit tests run using QUnit are split between: