var result = element.indeterminate; element.indeterminate = value;
The indeterminate property can be used to indicate whether the user has acted on a control. For example, setting the indeterminate to true causes the check box to appear checked and dimmed, indicating an indeterminate state. The value of the indeterminate property acts independently of the value of the checked property. Creating an indeterminate state is different from disabling the control. Consequently, a check box in the indeterminate state can still receive the focus. When the user clicks an indeterminate control, the indeterminate state turns off and the checked state of the check box toggles.
Microsoft Developer Network: [Windows Internet Explorer API reference Article]