JSFiddle - React, Tailwind, and code Playground

HTML

<p class="zoomable">Hi</p>
<p>Hi</p>
<p class="zoomable">Hi</p>
<p>Hi</p>
<a href="javascript:zoomIn()">Zoom In</a>

JavaScript

function zoomIn () {
 
  var elems = document.getElementsByClassName('zoomable');
 
  for(var i = 0; i < elems.length; i++) {
    var elem = elems[i];
    var fontSize = parseInt(window.getComputedStyle(elem, null).getPropertyValue('font-size'));
    elem.style.fontSize = (fontSize+1)+'px';
  }
 
}