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chromium/third_party/blink/web_tests/external/wpt/css/css-contain/reference/contain-paint-001-ref.html 

<!doctype html>
<html lang=en>
  <meta charset=utf-8>
  <title>CSS-contain test: paint containment use the padding edge</title>
  <link rel="author" title="Florian Rivoal" href="https://florian.rivoal.net">

<style>
div {
  width: 100px;
  height: 100px;
  background: blue;
  padding: 50px;
  border-radius: 100px;
  overflow: hidden
}
div::before {
  content:"";
  display: block;
  background: green;
  width: 100px;
  height: 100px;
}
</style>

<p>Test passes if there is a green square in a rounded blue box, and no red.
<div></div>