Set the width of the element's border to 10px.

<html>
<head>
<style>
.caution {
border: solid 10px red;
}
</style>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h1 class="caution">Anti-Vaccine Vaccine Discovered!</h1>
</body>
</html>]]>

Make sure to type 10px.