## TL;DR

ResizeObserver reports changes to the dimensions of DOM elements (content-box or border-box), replacing polling-based resize detection and `window.resize`-only approaches. It is essential for responsive components and container queries.

## Core Explanation

`new ResizeObserver(entries => { /* entries[0].contentRect */ })`. Different from `element.onresize` which only works on `window`. Observes actual element size changes including those caused by CSS, layout, or content changes. Runs after layout but before paint. Delivery is batched — multiple changes in the same frame are reported together.

## Further Reading

- [Resize Observer (W3C)](https://www.w3.org/TR/resize-observer-1/)