jQuery Simple Example

HTML

<script src="//www.addressfinder.co.nz/assets/v2/widget.js"></script>
<input type="number" min="0" max="100" value="100"> <span class="percentage-char">%</span>

CSS

input {
  width: 55px;
  height: 20px;
  font-size:18px;
}
.percentage-char {
    position: absolute;
    left: 32px;
    top: 1px;
    font-size: 18px;
}
.one-digit {
  left: 13px;
}
.two-digits{
  left: 24px;
}

JavaScript

$(":input").change(function() {
   if ($(this).val() < 10) {
   		$('.percentage-char').removeClass('two-digits');
   		$('.percentage-char').addClass('one-digit');
   } else if ($(this).val() > 9 && $(this).val() < 100) {
   		$('.percentage-char').addClass('two-digits');
   } else {
   		$('.percentage-char').removeClass('one-digit two-digits');
   }
});