css/css-values/cap-unit-001.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Values and Units Test: support for the cap unit</title>
<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
<link rel="help" href="https://drafts.csswg.org/css-values-4/#font-relative-lengths">
<link rel="match" href="reference/cap-unit-001-ref.html">
<meta name="assert" content="The cap unit is equal to the used cap-height of the first available font.">
<link rel="stylesheet" type="text/css" href="/fonts/ahem.css">
<style>
span {
    background: green;
    color: green;
    position: absolute;
}
div {
    font: 50px Ahem; /* cap-height of Ahem is 0.8em */
    background: red;
    position: relative;
    height: 180px;
    height: calc(180px - 2cap);  /* reduce to 100px if cap correctly supported */
    width: 100px;
}

div span {
    width: 2.5cap;
    height: 100px;
}
</style>
<body>
    <p>Test passes if there is a <strong>filled green square</strong> and <strong>no red</strong>.</p>
    <div><span></span></div>
</body>