<!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: fit-content; inline-size: 100px; border: 5px solid red; } </style> <div id="outer"><div style="block-size: 200px"></div></div>