At the heart of an EPUB lies Cascading Style Sheets (CSS) and Extensible HyperText Markup Language (XHTML). This grants the designer control over the look and feel of their eBooks, even in an environment where the reader can affect change at will.
The challenge in designing eBooks are the unknowns. It’s impossible to know what devices your readers will use and these uncertainties stack up. Some of these elements are:
- Screen size, resolution, and density. I.e., Dots Per Inch (DPI) or Pixels Per Inch (PPI);
- The margins and visible page size;
- Font type, spacing, and sizes;
- Zoom factor; and
- Orientation of the device.
Fonts can be embedded into an EPUB to control the look and feel. However, there is no guarantee that Amazon, Apple, Google, or Kobo will not strip them out in favour of their own. E.g., Bookerly on Kindle platforms.
It’s important that an EPUB reflow its content to adjust to changes in any of the above elements. A book that reflows ensures a quality reading experience for the reader.