Every self-publisher knows the frustration: your manuscript looks perfect in Word, but the exported ePub is a formatting disaster on different devices. Paragraphs break awkwardly, images overflow, and navigation fails. The core challenge is creating a truly reflowable book that adapts beautifully to any screen.

The Core Principle: Mobile-First, Relative Units

The key to universal compatibility is adopting a mobile-first CSS approach using relative units . Forget fixed, print-like measurements. Your CSS should instruct the reading system how elements relate to one another, not dictate exact pixels.

For example, using font-size: 12pt; margin-left: 50px; is a recipe for inconsistency. Reading systems and screens vary wildly. Instead, use relative units like rem and em . Define a base font size on the root element and scale everything from there: font-size: 1rem; margin-left: 2em; .…