JSFiddle - React, Tailwind, and code Playground

HTML

<div id="page-preloader"><span class="spinner"></span></div>
<div>content</div>

CSS

#page-preloader {
  position: fixed;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
  background: gray;
  z-index: 100500;
}

#page-preloader .spinner {
  width: 32px;
  height: 32px;
  position: fixed;
  left: 50%;
  top: 50%;
  background: green;
  margin: -16px 0 0 -16px;
  display: block;
}

JavaScript

$(window).on('load', function () {
    var $preloader = $('#page-preloader'),
        $spinner   = $preloader.find('.spinner');
    $spinner.fadeOut();
    $preloader.delay(350).fadeOut('slow');
});