index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">

  <title>Leaflet Test</title>

  <link rel="stylesheet" href="https://unpkg.com/leaflet@1.2.0/dist/leaflet.css"
   integrity="sha512-M2wvCLH6DSRazYeZRIm1JnYyh22purTM+FDB5CsyxtQJYeKq83arPe5wgbNmcFXGqiSH2XR8dT/fJISVA1r/zQ=="
   crossorigin=""/>
   <link rel="stylesheet" href="./stylesheets/main.css">
</head>

<body>
  <div id="main_map"></div>
</body>

<script src="https://unpkg.com/leaflet@1.2.0/dist/leaflet.js"
   integrity="sha512-lInM/apFSqyy1o6s89K4iQUKg6ppXEgsVxT35HbzUupEVRh2Eu9Wdl4tHj7dZO0s1uvplcYGmt3498TtHq+log=="
   crossorigin=""></script>
<script src="./scripts/main.js" type="text/javascript"></script>
</html>