How to track the content change of the object loaded via ajax?

Hello.
On the page you have an element into which loaded data from DB with some frequency.

<div id="answers"></div>

$("#answers").everyTime(10000, function() {
$.ajax({
 type: "POST", url: "update.php", data: "id="+zid+"",
 complete: function(data){
$("#answers").html(data.responseText);
}
});

How you can track changes in the content of an element inside <div id="answers"></div>?
For example if you change <div id="counter">8</div> to <div id="counter">9</div> display an alert with some text.

PS.: tried here so:

$('#counter').on('DOMSubtreeModified', function(){
 console.log('Content changed');
});

only responds when I change the value when updating is not working.

Now I read about MutationObserver, but saddened by the lack of support in IE below 11.