empty Div tag full size

hello all


i have a div tag inside my body tag.this div is empty and will remain empty throughout .also this div has a  transparent background using the opacity and background color attribute .
another div contains the contents.
when a button will be clicked from the contents div then the empty div will visible making content div unaccessible and i want to display another div above the empty div that contains some information and a close button.when the close button will clicked the empty div and div above it will hide and contents will be accessible again.It will be like a messagebox
but the problem is i can not set the width and height of empty div,i have set the width and height to 100% but in vain.Plz help me
i have also tried to google it but did not find any acceptable solution


thanks

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