Kobo and Adobe clash on ePub rendering, analysis shows

A new analysis of Kobo’s and Adobe’s ePub renderers shows that Adobe’s implementation deviates from the EPUB 3 specification in two key areas: font‑size declarations and margin‑collapse rules. The divergence produces noticeably different page layouts when the same ePub file is opened on a Kobo device versus Adobe’s Digital Editions or Acrobat Reader [hn-front].

Findings

• Font‑size handling – Adobe scales declared font-size values relative to the viewport, while Kobo applies them as absolute points. The result is larger text on Kobo screens for the same CSS rule.
• Margin collapse – Kobo respects the CSS margin‑collapse algorithm, merging adjacent vertical margins. Adobe treats each block’s margin independently, leaving extra whitespace in the flow.
• Image scaling – Adobe enforces a maximum image width of 100 % of the viewport, whereas Kobo allows images to exceed the viewport width if the author specifies larger dimensions, leading to overflow on Kobo devices.
• CSS support – Kobo implements the full EPUB 3 CSS spec for @font-face and font-variant, while Adobe falls back to system fonts when custom fonts are embedded.

Why it matters

These inconsistencies force e‑reader software engineers to maintain separate rendering pipelines or add extensive conditional logic to ensure a uniform reading experience. QA teams must validate each ePub on both platforms, increasing development cost and time‑to‑market. End users encounter broken layouts—text that is too large, unexpected gaps, or clipped images—depending on the device they choose, eroding confidence in the e‑book’s quality.

The analysis underscores the need for stricter adherence to the EPUB 3 spec across the industry. Until Adobe aligns its renderer with the specification, developers will continue to grapple with platform‑specific quirks.