OL2 Map with OSM

HTML

<link rel="stylesheet" href="http://dev.openlayers.org/releases/OpenLayers-2.13.1/theme/default/style.css">
<script src="http://openlayers.org/api/OpenLayers.js"></script>
<div id="map"></div>

CSS

#map {
    with:100%;
    height:300px;
}

JavaScript

// Set up the map
var map;
map = new OpenLayers.Map('map');
map.addLayer(new OpenLayers.Layer.OSM());

// Center to Spain (projecting from EPSG:4326 
var pointLonLat = new OpenLayers.LonLat(0, 0);
map.setCenter(pointLonLat.transform("EPSG:4326", "EPSG:3857"), 5);

// Add a layer
var markers = new OpenLayers.Layer.Markers("A tree");
map.addLayer(markers);
// Ad a marker to the layer
var tree = new OpenLayers.LonLat(-2.464459,36.83711);
markers.addMarker(new OpenLayers.Marker(tree.transform("EPSG:4326","EPSG:3857")));