If a ResizeObserver-based test fails, the problem is often not the browser but the measurement you asked for. ResizeObserverEntry exposes different boxes and units: contentBoxSize (content area), borderBoxSize (outer element footprint), and devicePixelContentBoxSize (content in device pixels). Each field answers a different diagnostic question.

Implementation decision

Start your test by naming the edge you intend to verify. The decision helper in the article is a tiny deterministic policy: it picks contentBoxSize when the question is about the available content area, borderBoxSize when the outer layout footprint matters, and devicePixelContentBoxSize when physical pixels matter.…