<style>

  input {

    margin-top: 1em;

    font-size: 14px;

    position: relative;

  }

  input::after {

    content: attr(data-value) '/' attr(max);

    position: absolute;

    left: 135px;

    top: -2px;

  }

</style>

​

<input type="range" min="0" max="100" value="25">

​

<script>

  var input = document.querySelector('input');

​

  input.dataset.value = input.value; // Set an initial value.

​

  input.addEventListener('input', function(e) {

    this.dataset.value = this.value;

  });

</script>