Allowing HTML in EPUBS

[sueneu]

I am reaching out here because we want to hear from people making EPUBs and making tools for EPUBs.

What if, in the future, EPUBs could contain HTML files in addition to XHTML?

As you know, technology is constantly evolving. One such technology, currently central to EPUB files, is XML. More exactly the XML syntax of HTML (XHTML). Use of this syntax is declining outside of publishing. In order to provide for the future of the EPUB ecosystem, we plan to remove the requirement for the XML syntax in the EPUB package.

This change will not affect the document formats currently allowed in an EPUB publication, it would simply remove the restriction to the XML syntax.

We hope to understand how this might affect the community. That way, we can identify potential obstacles and make an easier transition to accepting HTML.

Your input is valuable, and will help shape the future of ebooks.
Please take this brief four-question survey: https://www.w3.org/wbs/1/epubhtml/

• You do not need a W3C account
• Responses are anonymous
• The survey is in English and Japanese
• You may share this link with anyone you believe could give us a helpful perspective
• Survey closes in September
• The anonymized information we collect will be made public to ensure transparency in our process.

Who We Are
This survey was created by the Publishing Maintenance Working Group, part of the World Wide Web Consortium (W3C). Our mission is to maintain the Recommendations for EPUB files and ereaders. We work incrementally to improve the Recommendations, provide additional clarity, and grow the Recommendations for the future of Ebooks.

[KevinH]

I am already on record as being totally against adding html parsing rules to the epub3 standard since it will further fracture the epub3 base, will make backwards compatibility with old epub2 only readers next to impossible, and is just a really bad idea in any epub3 spec. Save it for epub 4, if you must.

Second, your proposition that xml is dying is simply incorrect and quite misleading. It should not be used in the lead in to your survey. Talk about adding a bias to a survey! Most current word processors (commonly used by fiction and non-fiction authors) use XML. Xml is also heavily used in text storage and archival and databases. Although html can omit some implied end tags, and handle some void tags differently, using open and closed tags is still part of the standard. Xhtml as a spec has long ago been replaced by xml parsing rules applied to html5 and its current living spec versions. This is not going to change no matter what the W3C thinks.

Also in the lead up to your survey link, you neglect to say this change would be made in the epub3 spec, and are thereby hiding the fractures and splits it would make to our current standard. I would have a very different response if this were being proposed for an epub 4 spec, as I suspect many others would.

[kovidgoyal]

I have to agree that this is rather unnecessary. XHTML as a spec might be dead, but using a valid XML serialization of HTML 5 is perfectly fine and supports evolving web standards perfectly. This is what happens in practice today anyway. Anything you can do in HTML 5 works fine when the HTML 5 is serialized as valid XML.

Forcing EPUB software developers to support HTML 5 parsing is just creating unnecessary busy work for no good reason.

The *only* advantage I can see for the HTML 5 serialization over the XML serialization is that the former is easier to write by hand. I dont think that is an advantage that justifies the cost. Most EPUB editing tools already have some kind of functionality to either flag or auto-correct invalid XML making writing it not that hard.

And I agree with KevinH that this disruptive of a change should be in EPUB 4 otherwise it will just end up getting ignored and un-used like the yoyo-ing that was done with EPUB metadata.

[Martinoptic]

I don't see what improvement it would make, and it would likely break backward compatibility with epub2 as Kevin says. Would readers want that - I think not. Why is there no question for readers rather than developers?
Edit: And why in an anonymous poll do you want my name, surname, employer and job title as well as my email address? Not very anonymous is it?

[DiapDealer]

I'm from the "Don't unnecessarily make API-breaking changes without a new major version (and a new soname)" camp myself. I know that analogy isn't exactly one-to-one, but it fits well enough.

But even that code of the school yard is being broken more and more often these days, it seems (*cough* libxml2 *cough*).

I did submit my feelings via the "anonymous" survey.

[PeterT]

There are some interesting discussions out there. See for instance "Allow pure HTML 5 in EPUB 3" https://www.edrlab.org/2025/07/06/al...ml5-in-epub-3/

[KevinH]

Interesting article. Yes a new standard would be a big improvement. Trying to shoehorn it into epub3 would not.

That said, Thorium will not work with pure html5 directly (I tested it) but it sounds like they are working on it.

[PeterT]

Another source worth following is the Public Mailing list of the "W3 Publishing Maintenance Group" especially the Agenda and Minutes from various meetings. See https://lists.w3.org/Archives/Public/public-pm-wg/

[kovidgoyal]

Originally Posted by KevinH

Interesting article. Yes a new standard would be a big improvement. Trying to shoehorn it into epub3 would not.

That's highly debatable. If a new standard is made by someone else than the people that gave us EPUB, maybe. Otherwise it will just be the same old mess hiding behind a new name.

That said, Thorium will not work with pure html5 directly (I tested it) but it sounds like they are working on it.

They appear to be doing some weird self contradictory things. First they say that the XML serialization of HTML 5 is fine and works in thorium. Then they go on to say that if an ebook has the non XML serialization for HTML 5 they will disable features such as pagination in their reader. I dont follow why the serialization format should make any difference to what features reader software supports. The actual DOM that is being serialized is independent of the format chosen for serialization and ebook reader features should depend on the DOM + CSS + JS not the serialization.

All-in-all it looks like there is a ton of confusion in this space. Starting with the people proposing this change who dont seem to understand that the serialization of HTML is irrelevant to what features it supports.

Like I said in my previous post the *only* pro for non XML serialization is that it is easier to author using a plain text editor.

[sueneu]

Thanks to everyone for replying, and thank you to those who filled out the survey.

I’ll be sure to share the anonymized results.

The survey is open until mid-September, so I’ll have complete results sometime after that.

[sueneu]

Originally Posted by Martinoptic

I don't see what improvement it would make, and it would likely break backward compatibility with epub2 as Kevin says. Would readers want that - I think not. Why is there no question for readers rather than developers?
Edit: And why in an anonymous poll do you want my name, surname, employer and job title as well as my email address? Not very anonymous is it?

Thank you for bringing this to our attention. The survey was made through the W3C’s survey system. We’ve sent a query to the system admins with this complaint. We will not publish people’s personal information when we release the results of the survey.

When you say users, do you mean people who use EPUB testing and editing tools, and/or user-readers who read Ebooks?

What question would you propose?

[Martinoptic]

Thank you for your reply. I was meaning people who read ebooks and may occasionally tweak them.
As I didn't get to the first question in the survey (as I'm not prepared to give all the details requested) it may well be that a further question isn't needed? As long as it isn't a requirement to have a job in e-publishing (which is what I took the sign up to suggest) then maybe the survey is ok as it stands. What do other people think?