<!DOCTYPE html>
<html lang="en-US">
<head>
<style>
body {
    width: 99999999px;
}
#test1 {
    font: 24px "system-ui";
}
</style>
<script>
function runTest1() {
    return document.getElementById("test1").offsetWidth;
}
</script>
</head>
<body>
<p>
<span id="test1">This is a test string</span>
</p>
</body>
</html>