<div class="alert">
  <span class="closebtn" onclick="this.parentElement.style.display='none';">&times;</span>
  This is an alert box.
</div>

<style>
.alert {
  opacity: 1;
  transition: opacity 0.6s; /* 600ms to fade out */
}
</style>

<script>
// Get all elements with class="closebtn"
var close = document.getElementsByClassName("closebtn");
var i;

// Loop through all close buttons
for (i = 0; i < close.length; i++) {
  // When someone clicks on a close button
  close[i].onclick = function(){

    // Get the parent of <span class="closebtn"> (<div class="alert">)
    var div = this.parentElement;

    // Set the opacity of div to 0 (transparent)
    div.style.opacity = "0";

    // Hide the div after 600ms (the same amount of milliseconds it takes to fade out)
    setTimeout(function(){ div.style.display = "none"; }, 600);
  }
}
</script>