A simple example of Leaflet.js using Mapbox tiles.
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<html lang="en">
<head>
<meta charset="utf-8">
<title>Leaflet with MapBox tiles</title>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/leaflet/0.7.7/leaflet.min.js"></script>
<link rel="stylesheet" href="https://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.css" />
<link rel="stylesheet" href="index.css">
</head>
<body>
<div id="map"></div>
<script src="index.js"></script>
</body>
</html>
Modified http://d3js.org/d3.v3.min.js to a secure url
Modified http://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js to a secure url
https://d3js.org/d3.v3.min.js
https://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js