Mt secret is to use <center> but I cant remember how many times I've considered using js and absolute positioning. The user clicks an image, the image is enlarg

Mt secret is to use <center> but I cant remember how many times I've considered using js and absolute positioning. The user clicks an image, the image is enlarged to fit the viewport, remaining space is filled with a color. Doable with css. The user zooms the image (scale), so far so good... the user pans the image... uhh how do I limit panning in css? bounding rectangle + get computered style??