<style>
body {
  margin: 0;
  --expected-block-size: 200;
}

.child {
  background: green;
}

@supports (display: layout(test)) {
  body {
    display: layout(test);
  }
}
</style>

<!-- In Quirks mode, we should stretch to 100% of the inital containing block. -->
<body>
<div class="child"></div>
</body>
