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<html><body>Checkbox: <input type="checkbox" id="myCheck"><p>Click the buttons to set or return the indeterminate state of the checkbox.</p><button onclick="myFunction()">Return state</button><button onclick="myFunction2()">Set state</button><p id="demo"></p><script>function myFunction() { var x = document.getElementById("myCheck").indeterminate; document.getElementById("demo").innerHTML = x;}function myFunction2() { var x = document.getElementById("myCheck").indeterminate = true;}</script></body></html>