Edit in JSFiddle

var map = L.map('map', {
  center: [53.2, -1.7],
  zoom: 6
});

L.tileLayer('https://osm.geolytix.net/osm_tiles/{z}/{x}/{y}.png').addTo(map);
html,
body {
  height: 100%;
  width: 100%;
  position: fixed;
  padding: 0;
  margin: 0;
  border: 0;
}

.map {
  height: 100%;
  width: 100%;
}
<div id="map" class="map"></div>