.className{
    width:300px;
    height:200px;
    position:absolute;
    left:50%;
    top:50%;
    margin:-100px 0 0 -150px;
}

$(window).resize(function(){

    $('.className').css({
        position:'absolute',
        left: ($(window).width() - $('.className').outerWidth())/2,
        top: ($(window).height() - $('.className').outerHeight())/2
    });

});

// To initially run the function:
$(window).resize();

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