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chromium/third_party/blink/web_tests/external/wpt/html/semantics/interactive-elements/the-dialog-element/centering-iframe.sub.html 

<!DOCTYPE html>
<meta charset=utf-8>
<title>dialog element centered frame</title>
<style>
  html {
    writing-mode: {{GET[html-writing-mode]}}
  }

  #container {
    writing-mode: {{GET[container-writing-mode]}}
  }

  dialog {
    writing-mode: {{GET[dialog-writing-mode]}};
    border: none;
    padding: 0;
    max-width: initial;
    max-height: initial;
    width: {{GET[dialog-width]}};
    height: {{GET[dialog-height]}};
  }
</style>

<div id="container">
  <dialog>X</dialog>
</div>

<script>
"use strict";
document.querySelector("dialog").showModal();
</script>