css/css-contain/contain-subgrid-001.html

<!doctype html>
<html lang=en>
  <meta charset=utf-8>
  <title>CSS-contain test: layout/paint containment on subgrid</title>
  <link rel="author" title="Mats Palmgren" href="mailto:mats@mozilla.com">
  <meta name=assert content="layout/paint containment inhibits subgrid">
  <link rel="match" href="reference/contain-subgrid-001.html">
  <link rel=help href="https://drafts.csswg.org/css-contain/#containment-layout">
  <link rel=help href="https://drafts.csswg.org/css-contain/#containment-paint">
  <link rel=help href="https://drafts.csswg.org/css-grid-2/">

<style>
.grid {
  display: grid;
  width: 40px;
  height: 40px;
  grid: [a] repeat(2,15px) [b] / [a] repeat(2,15px) [b];
  gap: 10px;
}

.subgrid {
  display: grid;
  grid: subgrid;
  background: lightgrey;
  grid-area:1/1/3/3;
}

.layout { contain: layout; }
.paint { contain: paint; }

.inner {
  background: blue;
  grid-area:a/a/b/b;
}
.p {
  place-self: start;
  width:100%;
  height:100%;
}
</style>

<div class="grid"><div class="subgrid layout"><div class="inner"></div></div></div>
<div class="grid"><div class="subgrid paint"><div class="inner"></div></div></div>
<div class="grid"><div class="subgrid layout"><div class="inner p"></div></div></div>
<div class="grid"><div class="subgrid paint"><div class="inner p"></div></div></div>