jQuery.on(“drop”) not firing

You need to cancel all events

$("html").on("dragover", function(event) {
    event.preventDefault();  
    event.stopPropagation();
    $(this).addClass('dragging');
});

$("html").on("dragleave", function(event) {
    event.preventDefault();  
    event.stopPropagation();
    $(this).removeClass('dragging');
});

$("html").on("drop", function(event) {
    event.preventDefault();  
    event.stopPropagation();
    alert("Dropped!");
});