Ubivectel,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<script>
$(function(){
var current = Number(localStorage.getItem("font") || 18);
function setSize(n)
{
if(n === '') current = 18;
else current += n;
localStorage.setItem("font", current.toString());
var font = {'font-size' : `${current}px`}
$('#KIDE_output, .ulcontent, #KIDE_txt').css(font);
}
setSize(0);
// reset
$(".resetMe").click(function(){
setSize('')
});
// Increase Font Size
$(".increase").click(function(){
setSize(1)
});
// Decrease Font Size
$(".decrease").click(function(){
setSize(-1)
});
});
</script>
</head>
<body>
<button class="resetMe">resetMe</button>
<button class="increase">increase</button>
<button class="decrease">decrease</button>
<div id="KIDE_output">KIDE_output</div>
<div id="KIDE_txt">KIDE_txt</div>
<div class="ulcontent">ulcontent</div>
</body>
</html>