Inksky
EPUB 2 gave us reflowable text. EPUB 3 gave us books that can read themselves aloud, footnotes that pop up instead of sending you to the end of the chapter, and equations that render like actual math instead of fuzzy images. Most ebooks today are EPUB 3 under the hood — the spec was finalized in 2011 — but support for the advanced features is wildly inconsistent across devices. Here’s what’s actually usable in 2026.
Media Overlays: synced audio and text
Media overlays let an EPUB synchronize narrated audio with highlighted text, turning the book into a read-along experience. Think of it as an audiobook where the words light up as they’re spoken. It’s built on SMIL (Synchronized Multimedia Integration Language), and authoring it requires pairing each paragraph or word with an audio clip.
Device support: iBooks (macOS/iOS) handles media overlays beautifully — it’s the gold standard. Thorium Reader on desktop supports them. Kindle ignores them entirely — Amazon’s Immersion Reading uses a separate, proprietary system. Kobo doesn’t support them. Boox running the Kindle app gets Immersion Reading via Amazon’s system, but no EPUB-level media overlays.
When to use them: children’s read-along books, language learning (hearing pronunciation while seeing the word), and accessibility (dyslexic readers benefit enormously from synced text). For standard novels, the production cost outweighs the benefit.
Pop-up Footnotes
EPUB 3’s <aside> element with epub:type="footnote" lets footnotes appear as a pop-up overlay instead of jumping to an endnote section and losing your place. Tap the footnote marker, read the note in a small overlay, tap outside to dismiss.
Device support: Apple Books, Kobo (KEPUB format only — standard EPUB uses end-of-chapter notes), and KOReader handle pop-up footnotes well. Kindle converts footnotes to endnotes during delivery and uses its own inline pop-up system, which works fine but isn’t technically EPUB 3 footnotes. Boox’s NeoReader supports them natively. PocketBook handles them inconsistently.
The catch: many publishers still format footnotes the EPUB 2 way (as end-of-book links) because it works everywhere. Pop-up footnotes only work if both the ebook is authored correctly AND the reading app supports them.
MathML: real equations, not images
MathML embeds mathematical expressions as structured markup rather than images. The equation scales with your font size, renders crisply at any resolution, and is accessible to screen readers. EPUB 3 includes MathML as a core specification.
Device support: Apple Books and Thorium Reader handle MathML natively. Kindle converts MathML to SVG images during ingestion — usable but loses reflow and accessibility. Kobo’s RMSDK renderer has limited MathML support; KEPUBs fare slightly better. Boox devices running KOReader get full MathML. PocketBook does not support MathML.
When to use it: STEM textbooks, academic papers, and any book with significant math content. For a novel with one equation, an image is fine. For a calculus textbook, MathML is essential for readability.
Other EPUB 3 features worth knowing
- Fixed-layout EPUB 3: for children’s picture books and graphic novels where every page needs exact positioning. Supported on nearly everything.
- Semantic markup: EPUB 3’s
epub:typeattribute labels content as chapters, forewords, glossaries, etc., improving navigation and accessibility. Supported widely. - JavaScript interactivity: EPUB 3 allows JavaScript for interactive quizzes and widgets. iBooks supports it; most e-ink devices don’t.
The pragmatic approach
Author in EPUB 3. The spec is mature, and the basic features — reflowable text, embedded fonts, SVG images — work everywhere. For advanced features, know your audience’s device. If your readers are primarily on Kindle, invest in Kindle-specific enhancements (Enhanced Typesetting, Immersion Reading) rather than EPUB 3 features Amazon ignores. If your readers use Apple Books or KOReader on Boox, EPUB 3’s full toolbox is available.