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<!DOCTYPE html>
<meta charset="utf-8">
<title>Test reference</title>
<style>
body > div {
font-family: monospace; /* to be able to reliably measure things in ch*/
font-size: 20px;
color: transparent;
writing-mode: vertical-rl;
background: green;
height: 1ch;
}
</style>
<p>Test passes if there is a <strong>green rectangle</strong> below and <strong>no red</strong>.
<div>0</div>