<!DOCTYPE html>
<html>
<head>
<title>Title of the document</title>
<style>
input:indeterminate {
box-shadow: 0 0 2px 2px #666;
}
</style>
</head>
<body>
<h2>Indeterminate selector example</h2>
<form>
<input type="checkbox" id="box"> Checkbox
<script>
var checkbox=document.getElementById("box");
checkbox.indeterminate=true;
</script>
</form>
</body>
</html>