The contentBoxSize read-only property of the ResizeObserverEntry interface returns an array containing the new content box size of the observed element when the callback is run.

An array containing objects with the new content box size of the observed element. The array is necessary to support elements that have multiple fragments, which occur in multi-column scenarios. Each object in the array contains two properties:

blockSize

The length of the observed element's content box in the block dimension. For boxes with a horizontal writing-mode, this is the vertical dimension, or height; if the writing-mode is vertical, this is the horizontal dimension, or width.

inlineSize

The length of the observed element's content box in the inline dimension. For boxes with a horizontal writing-mode, this is the horizontal dimension, or width; if the writing-mode is vertical, this is the vertical dimension, or height.

The following snippet is taken from the resize-observer-border-radius.html (see source) example. This example includes a green box, sized as a percentage of the viewport size. When the viewport size is changed, the box's rounded corners change in proportion to the size of the box. We could just implement this using border-radius with a percentage, but that quickly leads to ugly-looking elliptical corners; this solution gives you nice square corners that scale with the box size.

const resizeObserver = new ResizeObserver((entries) => {
  for (let entry of entries) {
    if (entry.contentBoxSize) {
      // The standard makes contentBoxSize an array...
      if (entry.contentBoxSize[0]) {
        entry.target.style.borderRadius =
          Math.min(
            100,
            entry.contentBoxSize[0].inlineSize / 10 +
              entry.contentBoxSize[0].blockSize / 10,
          ) + "px";
      } else {
        // ...but old versions of Firefox treat it as a single item
        entry.target.style.borderRadius =
          Math.min(
            100,
            entry.contentBoxSize.inlineSize / 10 +
              entry.contentBoxSize.blockSize / 10,
          ) + "px";
      }
    } else {
      entry.target.style.borderRadius =
        Math.min(
          100,
          entry.contentRect.width / 10 + entry.contentRect.height / 10,
        ) + "px";
    }
  }
});

resizeObserver.observe(document.querySelector("div"));