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chromium/chrome/test/data/android/display_cutout/subframe.html 

<!DOCTYPE html>
<meta name="viewport" content="width=device-width initial-scale=1.0 viewport-fit=cover" />
<script src="shared.js"></script>
<style>
  #target {
    margin-top: env(safe-area-inset-top);
    margin-left: env(safe-area-inset-left);
    margin-bottom: env(safe-area-inset-bottom);
    margin-right: env(safe-area-inset-right);
  }
</style>
<div id=target></div>