Digital Publishing: Are You Getting the Most Out of EPUB 3?

More Than a Website in a Box

Because EPUB is a single file format (.epub), and because it is fundamentally based on modern Web standards (HTML5, CSS3, and JavaScript), people often think of EPUB 3 as “just a website in a box” that can be viewed offline. Actually, it’s a whole collection of files — content documents, style sheets, fonts, images, media resources, scripts, metadata and more — that is literally zipped up (an .epub is a type of .zip file) for reliable single-file delivery. More importantly, it’s an organized collection of files, governed by a “package file” that documents what the EPUB contains (in the manifest), what a reading system needs to know about the EPUB and everything in it (in the metadata), a default reading order (the spine) and so forth.

Still, a publication starts with the content, and true to EPUB 3’s Web-based foundation, EPUB 3 content documents are almost always XHTML5 files. (XHTML is HTML using the rules of XML; more about that below.) They can also be SVG — a vector- and XML-based image format — but there are few SVG-based reading systems. As those come along, we’ll see SVG used for manga, comics, graphic novels and other image-based publications. But for now, the focus is on XHTML5.

Building on the Built-in Semantics of XHTML5

There are two essential things that differentiate HTML5 from its predecessors: It eliminates presentational markup, or formatting (that’s handled by your CSS, or cascading style sheet, instead) and it provides specific semantics for structural elements. Where XHTML 1.1 uses general purpose container elements like