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