Using post navigation with CSS classes instead images

In WordPress, I have created a custom post type. For navigation I created a left and a right arrow with pure CSS, .arrow_left and .arrow_right.

My CSS:

.arrow_left {
  position:absolute;
  left:10px;
  top:5px;
  width: 0;
  height: 0;
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent;
  border-right:10px solid lightgrey;
}

.arrow_right {
  position:absolute;
  right:10px;
  top:5px;
  width: 0;
  height: 0;
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent;
  border-left:10px solid lightgrey;
}

Now I would like to attach the navigation functionality to these CSS arrows. I tried out the following:

<?php previous_post_link('%link','<div class="arrow_left"></div>'); ?>
<?php next_post_link('%link','<div class="arrow_right"></div>'); ?>

My problem: When applying this code, both arrows disappear. What’s wrong about the code?