window.addEventListener("scroll",onScroll);
function onScroll() {
    'use strict';
    var boxes = document.getElementsByClassName("infoboxbackground");
    for(var i = 0; i < boxes.length; i++) {
        var box = boxes[i].getBoundingClientRect();
        boxes[i].className = boxes[i].className.replace(" active", "");
        if(box.top < window.innerHeight * 0.6 && box.bottom > window.innerHeight * 0.4) {
            boxes[i].className += " active";
        }
    }
}