CodePen Embed - ResizeObserver

<div class="boxes">
  <div class="box"></div>
  <div class="box"></div>
</div>

.boxes {
  display: flex;
  flex-wrap: wrap;
  width: 40vw;
  margin-bottom: 50px;
}
.box {
  height: 200px;
  flex-shrink: 1;
  flex-grow: 1;
  margin: 10px;
  box-sizing: border-box;
  border: 3px solid black;
}

const ro = new ResizeObserver(entries => {
  for (let entry of entries) {
    entry.target.style.borderRadius =
      Math.max(0, 250 - entry.contentRect.width) +
      'px'
  }
})
console.log('horay');
// Only observe the 2nd box
ro.observe(
  document.querySelector('.box:nth-child(2)')
)

This Pen is owned by Yaser Adel Mehraban on CodePen.

See more by @yashints on CodePen

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.