:indeterminate CSS Pseudo Class

<div>

  <input type="checkbox" id="checkbox">

  <label for="checkbox">This label starts out green.</label>

</div>

<br>

<p>Progress bar without a value; i.e. in an indeterminate state.</p>

<progress>

<script>

var inputs = document.getElementsByTagName("input");



for (var i = 0; i < inputs.length; i++) {

  inputs[i].indeterminate = true;

}

</script>

input:indeterminate + label {

   background: green;

}

progress {

   margin: 4px;

}

progress:indeterminate {

   border: 1px solid green;

}