[Quoth]

Calibre Viewer is using some sort of QT webkit browser, nothing like the renderers in most ebooks, so only use to check TOC makes sense and all links (internal or external) work.
It's fine for people that don't have phone, tablet or eink to read on, but it will not behave the same as real ereaders with exotic HTML or CSS. Some Android Apps might work similarly.

An iphone / iPad may only have HTML3 / old mobi rendering (kindle app) or a renderer based on whatever species of Safari is built in. It's a separate ecosystem, certainly ebooks bought from Apple books using epub2 source are not shown the same as epub loaded direct to Apple Books. The iOS Kindle doesn't render the same as Android Kindle, it's more like Old Mobi.
Apple automatically converts epub2 loaded to their store to be 100% Books compatible. Directly (sideloaded) epub on Books (formerly iBooks) needs extra code.