If you are using the ModalPopup extender control from the Ajax Control Toolkit you might have noticed that the popup flickers when the page is loaded. This is because of that there is a slight delay from when the page is rendered to when the javascript of the extender control hides the popup.
To get around this you need to set the display style of the popup element. It seems like you have to set the style directly at the control, and not from an external style sheet.
To set the display style to none for your popup element, add the following attribute:
style="display: none"
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