chromium/third_party/blink/web_tests/compositing/overflow/siblings-with-border-radius-ancestor.html

<style>
.outermost {
  border: 4px solid #000;
  border-radius: 40px;
  width: 200px;
  height: 200px;
  overflow: hidden;
}

.inner1 {
  background-color: green;
  width: 10px;
  height: 260px;
  position: relative;
  left: 0px;
  top: -30px;
  will-change: transform;
}

.inner2 {
  background-color: green;
  width: 10px;
  height: 260px;
  position: relative;
  left: 190px;
  top: -260px;
}
</style>

<div class="outermost">
  <div class="inner1"></div>
  <div class="inner2"></div>
</div>