As you already know, everyone has a different graphics resolution on their PC. So how do you center a pop-up window for all resolutions. It is easier than you think.
Lets say you want to open a window that has a size of 200h x 150w. By taking half of the height and width (100 and 75 respectively) you can pinpoint the center of the window. Then with a little math you can center the window. Here is how it is done
To call this function in HTML use this: