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chromium/third_party/blink/web_tests/shadow-dom/ua/meter-fallback.html 

<!DOCTYPE html>
<meter style="-webkit-appearance: none">A<span slot="x">B</span><span>C</span></meter>
<div id="host"><span>X</span></div>
<script>
'use strict';

host.attachShadow({mode: 'open'}).innerHTML =
  `<meter style="-webkit-appearance: none"><slot></slot></meter>`;
</script>