Edit in JSFiddle

var map = L.map('map').setView([48.845,2.424], 10);

L.tileLayer('http://a.tile.openstreetmap.org//{z}/{x}/{y}.png', {
    attribution: 'Map Quest ©',
    maxZoom: 18,
}).addTo(map);

<div id="map" class="map"></div>

.map {
  height: 400px;
  width: 100%;
}