JSFiddle - React, Tailwind, and code Playground

HTML

<div id="text">
  Bardzo ważny tekst
</div>
<div class="buttons">
  <button id="bigger">+</button>
  <button id="smaller">-</button>
</div>

JavaScript

var text = document.getElementById("text");
var bigger = document.getElementById("bigger");
var smaller = document.getElementById("smaller");



function makeBigger() {
    var fontSize = parseInt(window.getComputedStyle(text).fontSize);
    text.style.fontSize = (fontSize+2) + "px";
}

function makeSmaller() {
    var fontSize = parseInt(window.getComputedStyle(text).fontSize);
    text.style.fontSize = (fontSize-2) + "px";
}



bigger.onclick = makeBigger;
smaller.onclick = makeSmaller;