JSFiddle - React, Tailwind, and code Playground

HTML

<a href="#" data-size="big" class="font_resize">bigger</a><br> 
<a href="#" data-size="small" class="font_resize">default</a><br> 


texttexttexttext<br>

CSS

html { font-size:13px; }
html.big { font-size:18px; }
html.default { font-size:13px; }

JavaScript

$(function() { 
  $('.font_resize').on('click', function() {
    $('html')
      .attr('class', '')
      .addClass($(this).data('size'));
  }); 
});