indeterminate(:indeterminate)

The :indeterminate CSS pseudo-class represents any <input type=”checkbox”> element whose indeterminate DOM property is set to true by JavaScript. In addition, in some browsers, it can be used to match to <progress> elements in an indeterminate state.

Example

<!DOCTYPE html>
<html>
    <head>
        <style>
            input, span {
                background: red
            }:indeterminate,:indeterminate + span {
                background: limegreen
            }
        </style>
    </head>
    <body>
        <p>
            <input type=checkbox><span>
            Everything in this paragraph should have a green background.
            </span>
        </p>
        <script>
            document.getElementsByTagName("input")[0].indeterminate = true;
        </script>
    </body>
</html>