<html>
<head>
<style>
input:indeterminate {
  opacity: 0.5;
}

input:checked {
  opacity: 1.0;
}
</style>
</head>
<body>
<input type=checkbox id="one"> This checkbox should look 50% transparent and should
be in the mixed state.
<script>
document.getElementById('one').indeterminate = true
</script>
</body>
</html>