DIAGRAM Center Best Practice Recommendations for EPUB 3: Now and Looking to the Near Future

Editor: George Kerscher

A DIAGRAM Center Standards and a Transition to Accessible EPUB Working Group Report


The DIAGRAM Center is wrapping up a five-year funding cycle. We have made tremendous progress in developing recommendations for MathML, chemistry, and for Extended descriptions sometimes referred to as long descriptions. However, technology continues to move forward, and it will be necessary for the accessibility community to monitor developments and update these recommendations from time-to-time.

The DIAGRAM Center Standards Working group and the Transition to Accessible EPUB 3 Working group (TIES), under the management of the DAISY Consortium, have been holding joint meetings. Moving forward, we will be inviting any of the DIAGRAM Center Working Group members to join TIES to continue the monitoring of this work and to take on new activities as the need arises.


The DIAGRAM Center and DAISY best practice interim recommendation for EPUB 3 can be found at: http://diagramcenter.org/best-practices-for-authoring-math.html

The DAISY Knowledge Base recommendation can be found at: http://kb.daisy.org/publishing/docs/html/mathml.html

The Near Future – MathML

MathML 3.0 has been around for a long time and implementations have been limited, with MathJax being the most notable. As a result, there is an initiative to take a close look at MathML to see what may be improved to make it easier to implement and improve adoption. The MathML Refresh Community Group (CG) can be found at: https://www.w3.org/community/mathml4/

The MathML Refresh CG is currently working on a W3C Charter for a MathML 4 revision. In addition, there is a campaign to raise funds to implement the MathML specification in Chromium so that it would be easy to include it in the Chrome and Edge browsers. Igalia has been working on this code and today in both Chrome and Edge, MathML can be enabled in the experimental version.

A recent email from Neil Soiffer below:

The Igalia work continues to be slowly added to the developer version of Chrome. You can see it in Chrome Canary (the nightly builds) if you enable chrome://flags/#enable-experimental-web-platform-features. You can do this in your regular Chrome browser also, although that shows you the state of MathML support a couple of months back. Same is true for Edge (edge://flags/#enable-experimental-web-platform-features). There are maybe five big features left to get into Chrome before I would say the math layout is good, but right now, with a couple of exceptions, most people would think it is fine.

Getting MathML into Chrome changes nothing about accessibility. It is still MathML and all the screen readers and other AT that support it will work just fine with it. Using your AT, you probably won’t notice any difference if you turn on/off the experimental flag; sighted users will see a big difference though. For the last few years, Chrome has been in the odd position that the math is more accessible to screen reader users than it is to sighted users. Now Edge is in that position also.

End quote.

To learn more visit: https://mathml.igalia.com/

What Does This Mean

It is expected that what is now only in the experimental version of Chrome and Edge will be moved into the core of Chromium and WebKit. MathML will then be able to be natively displayed in browsers. More Math should then be presented on the Web. EPUB reading systems, which mostly use the underlying web browser components to render content in the reading systems would begin to get MathML support for free. It will take time for the reading systems to upgrade to this new code base, but it should come fairly rapidly. We need to continue to track the implementations of EPUB 3 reading systems, and it may be that we will have native support for MathML in most reading systems. At that point our recommendation would change and only recommend the use of MathML and avoid the use of an image with alt text.

Extended Descriptions (Long Descriptions)

Images and other types of content may require a description that is too long to fit in alt text and may require the use of markup to best present this information. For example, a pie chart can present visually information, but be inaccessible to persons with disabilities. Placing the same information in a table can make the information accessible to persons with disabilities. However, this table would be redundant information and can disrupt the flow of the page. For this reason, we have developed techniques for presenting extended descriptions in less intrusive ways.

The best practice recommendation for extended description in EPUB 3can be found at: http://diagramcenter.org/best-practices-for-authoring-extended-descriptions.html

In addition, the recommendation for images and extended descriptions in the DAISY Knowledge Base can be found at: http://kb.daisy.org/publishing/docs/html/images.html

The Near Future – Extended Descriptions

The two issues we found with the techniques are: The “details” element is not fully supported in some reading systems,

  • The technique of linking to the extended description in a separate file may present problems coming back to the exact location of the launching link.
  • We are optimistic that reading systems will improve with both issues based on our testing and reporting the problems to developers of the reading systems.


A W3C Community Group called “Chemistry for the Web and in Publishing” has been formed and can be found at: https://www.w3.org/community/chem-web-pub/

This CG has worked closely with the MathML Refresh CG to identify best practices for using MathML to present chemical expressions. A method to indicate that a MathML instance represents chemistry is being added to MathML, and mechanisms for the correct pronunciation are also being defined.

The DIAGRAM Center report on the activities of the CG, which has more detail can be found at: http://diagramcenter.org/diagram-center-report-on-chemistry.html

The Near Future – Chemistry

As the MathML 4 work progresses in the CG and perhaps in the proposed Working Group, we can expect that the semantics of chemistry will be added to chemical expressions. The notion of “author intent” is now being discussed in the CG, which would encompass semantics. However, this will require authors and publishers to add additional information to the MathML. Because authoring tools are normally used for creating MathML, it will be necessary for these authoring tools to be upgraded to add this additional information. Some math tools already have a Chemistry mode so that chemical elements like “H” are not display in an italic font as they would be in mathematics. Adding the extra information that this represents chemistry should be very simple. When this will happen is unknown, but the semantics in the MathML will need to be defined first. It should also be noted that very interesting work is being done with SVG to present molecular diagrams that present detailed information to persons with disabilities. More information on “Progressive Chemical Diagrams” can be found at: https://www.progressiveaccess.com/chemistry/index.html

Final Thoughts

Finally, this working group wants to reiterate the need for significant funding to implement and expand the developments that are taking place in the Chemistry CG and in the MathML CG and hopefully the proposed WG. We appreciate the volunteers in these groups, but unless serious funding is made available, the progress will be very slow.

Last updated: October 9, 2020

Ideas that work.The DIAGRAM Center is a Benetech initiative supported by the U.S. Department of Education, Office of Special Education Programs (Cooperative Agreement #H327B100001). Opinions expressed herein are those of the authors and do not necessarily represent the position of the U.S. Department of Education.


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