How HTML changes in ePub

Summary

How HTML changes in ePubby Robin Whittleton published on Dec 11, 2025Skip to commentsePub is the W3C standard for ebooks. It lets you take your knowledge of the web, and use it to produce little self-contained sets of documents that can be freely distributed as a single file ready for reading on extremely low-power devices, and they even reflow to fit any screen.Yet while I said that you can use your knowledge of the web to build ePubs, the technology in use is twisted in unforeseen ways, and you might have to unlearn the things you think you knew. Prepare yourself…HTML, sort ofePubs, at their core, use HTML, just like the websites we build every day. Except, well, there’s a big asterisk after that. Let’s dive into the differences.A few decades ago XML emerged from the pit. XML – an extensible standard for expressing marked up data – could be used for documents, data transfer, and a bunch of other things, and people genuinely liked it (or, much like AI today, pretended to for job security). They liked it so much that a concerted effort was started to take HTML and rebuild it on top of XML. This project had a name you might have heard of: XHTML.XHTML didn’t work out, for a number of reasons. The extensibility of XML turned out to not be useful when browsers didn’t support even common extensions. Then there was the problem of fragility: any syntax problems with your XHTML and your users would get a blank screen. If those two problems weren’t enough, XHTML was slower in practice because the browser needed to wait to download the entire document before doing anything else.But there is one place where XHTML still rules the roost: ePub. ePub books are, at their heart, a collection of XHTML documents (now using the XHTML flavour of the HTML Living Standard). This means that:Valid, syntactically correct XML markup is needed. Without that, your e-reader will complain. This means self-closing tags, correct namespaces, XML attributes in the XML namespace (xml:lang), and so on....