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chromium/third_party/blink/web_tests/external/wpt/css/css-page/page-box-001-print.html 

<!DOCTYPE html>
<link rel="author" title="Morten Stenshorne" href="mailto:[email protected]">
<link rel="help" href="https://drafts.csswg.org/css-page-3/#page-properties">
<link rel="match" href="page-box-001-print-ref.html">
<style>
  @page {
    margin: 0;
    background: yellow;
  }
  body {
    margin: 100px;
  }
</style>
The entire page should be yellow.