I have created a checkbox as follows:

	<ToggleButton class="Checkbox" checked="true" onactivate="OnCheckboxClicked()">		
		<Panel />
But the actual style is not Checkbox:selected, until I set the checked property of the togglebutton manually in javascript. If I explicitly set checkbox.checked = true in javascript, the css works properly, otherwise the checkbox always starts in the deselected state / unchecked state.