css/css-sizing/div-max-content-orthogonal-auto-margin.tentative.html

<!DOCTYPE HTML>
<title> Ensure element is centered by auto margin for orthogonal writing mode </title>
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/3973">
<link rel="match" href="div-orthogonal-auto-margin-ref.html">
<style>
  #outer {
    inset: 0;
    background-color: black;
    writing-mode: vertical-rl;
    margin: auto;
    position: absolute;
    block-size: max-content;
    inline-size: 100px;
    border: 5px solid red;
  }
</style>
<div id="outer"><div style="block-size: 200px;"></div></div>