Why does my EPUB look different on different devices?

That's not a flaw — it's what an ebook is supposed to do. Unlike a fixed PDF, an EPUB reflows: each reader picks their own font, text size, line spacing, and screen, and your book rearranges to fit. So the same EPUB genuinely should look different on a small phone, a large tablet, and an e-ink Kindle.

What stays consistent is the structure and meaning — chapters, paragraph breaks, italics, and bold — not the exact page breaks or line endings. Don't try to force a fixed "page" look in an ebook; readers expect and want to control the presentation. Just confirm it reflows cleanly at a few font sizes — see How do I test the EPUB?