Everything you need to know about EPUB
EPUB (Electronic Publication, .epub) is the open ebook standard maintained by the W3C-managed International Digital Publishing Forum since 2007. Under the hood it's a glorified ZIP file containing HTML, CSS, and metadata - making it the ebook format you can actually edit by hand if you want to.
How it works under the hood
- It's a ZIP. Rename any `.epub` to `.zip` and you can extract its guts: an `OEBPS/` folder with chapter HTML files, a `META-INF/container.xml` pointer, and a `content.opf` manifest listing every resource.
- Reflowable by default. Unlike PDF, EPUB content reflows to fit any screen size. The reading device controls font, size, line spacing, and theme - the file just provides semantic HTML structure.
- EPUB 3 = HTML5. EPUB 3 (current standard) embraced HTML5, CSS3, JavaScript, MathML, and SVG. Fixed-layout EPUB 3 supports comics and children's books with pixel-precise positioning.
- Accessibility-first. EPUB has native ARIA support, alt text, semantic landmarks, and read-aloud (Media Overlays / SMIL). It's the only ebook format with serious accessibility guarantees.
Where you'll actually use it
- Apple Books, Kobo, Nook, Google Play Books (the universal e-reader format)
- Self-publishing on retailers that aren't Amazon Kindle
- Long-form web content distributed offline (manuals, documentation)
- Accessible educational publishing (DAISY consortium uses EPUB 3 as base)
How it compares to alternatives
EPUB vs MOBI: MOBI is Amazon's older proprietary format - reflowable but less feature-rich, replaced by AZW3. EPUB vs PDF: EPUB reflows; PDF doesn't. Read EPUB on phones, PDF on bigger screens. EPUB vs AZW3: AZW3 is essentially EPUB with Amazon DRM and a different file extension.
Things that will trip you up
- Validators care about strict OPF schema compliance - run `epubcheck` before publishing or you'll get rejected from Apple Books
- Fixed-layout EPUB doesn't reflow - test on every target device size before shipping
- Embedded fonts must be obfuscated per IDPF spec for some retailers - Calibre and Sigil handle this for you
Test it yourself: `epubcheck` (W3C official validator), Calibre for inspection and conversion, Apple Books and Adobe Digital Editions for cross-device testing.