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chromium/third_party/blink/web_tests/external/wpt/css/css-contain/reference/contain-body-w-m-001-ref.html 

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

<style>
html::before {
    content: "";
    width: 100px;
    height: 100px;
    background: orange;
    display: block;
}
body { margin: 0; }
p {
    margin: 0;
    width: 200px;
    height: 200px;
}
</style>

<p>Test passes if the orange square is in the upper-left corner.