Leaflet.Spin standalone (similar to official https://github.com/makinacorpus/Leaflet.Spin but use only CDNs)

<!DOCTYPE html>
<html>
<head>
  <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  <title>Leaflet.Spin</title>
  <link rel="stylesheet" href="https://unpkg.com/leaflet@1.4.0/dist/leaflet.css" />
  <script src="https://unpkg.com/leaflet@1.4.0/dist/leaflet.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/spin.js/2.3.2/spin.min.js" charset="utf-8"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/leaflet.spin/1.1.0/leaflet.spin.min.js" charset="utf-8"></script>
  <style type='text/css'>
    #map {
        position: absolute;
        top:0;
        left: 0;
        right: 0;
        bottom:0;
    }
    #btn-spin {
      position: absolute;
      left: 200px;
      z-index: 10;
      font-size: 1.5em;
      z-index: 3000;
    }
    .help {
        font-size: 1.5em;
        position: absolute;
        top:0;
        left: 0;
        right: 0;
        height: 30px;
        z-index: 10;
        background-color: rgba(0,0,0,0.5);
        color: white;
        padding: 10px;
        margin: 0px;
        text-align: center;
        z-index: 3000;
    }
    .help a.sources {
        float: left;
        margin-left: 50px;
        color: white;
    }
    .help a.logo {
        float: right;
    }
    .help a.logo img {
        height: 30px;
    }
  </style>
</head>
<body>
  <p class="help">Leaflet + Spin.js
      <a class="sources" href="https://github.com/makinacorpus/Leaflet.Spin/">Sources</a>
      <a class="logo" href="https://makina-corpus.com"><img src="https://depot.makina-corpus.org/public/logo.gif"/></a>
  </p>
  <div id="map"></div>

Updated missing url https://cdnjs.cloudflare.com/ajax/libs/Leaflet.Spin/1.1.0/leaflet.spin.min.js to https://cdnjs.cloudflare.com/ajax/libs/leaflet.spin/1.1.0/leaflet.spin.min.js

https://unpkg.com/leaflet@1.4.0/dist/leaflet.js

https://cdnjs.cloudflare.com/ajax/libs/spin.js/2.3.2/spin.min.js

https://cdnjs.cloudflare.com/ajax/libs/Leaflet.Spin/1.1.0/leaflet.spin.min.js