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chromium/third_party/blink/web_tests/external/wpt/infrastructure/assumptions/medium-font-size.html 

<!doctype html>
<title>Check medium font-size is 16px</title>
<link rel="match" href="medium-font-size-ref.html">
<style>
p {
  font-size: medium;
}
</style>
<p>This text should be 16px