'indeterminate' Example
<html> <body> <input type="checkbox" id="myCheckBox" value="checkbox" checked> Indeterminate = true. <input type="button" value="Checkbox state" onClick="function1();"> <script language="JavaScript"> document.all.myCheckBox.indeterminate = true; function function1() { var m = document.all.myCheckBox.indeterminate; alert("Indeterminate: "+m); } </script> </body> </html>