css/css-backgrounds/background-margin-iframe-root.html

<!doctype HTML>
<title>Backgrounds on the root element of an iframe should extend to cover the entire canvas of the iframe in the presence of margin</title>
<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
<link rel="help" href="https://www.w3.org/TR/css-backgrounds-3/#root-background">
<link rel="match" href="background-margin-iframe-root-ref.html">
<meta name=fuzzy content="0-1;0-750">
<iframe srcdoc="
<!doctype HTML>
<style>
  html {
    background: linear-gradient(lightblue, yellow);
    height: 300px;
    margin: 50px;
    overflow: hidden;
  }
</style>"></iframe>