Edit in JSFiddle

var map = L.map('map').setView([65.276178, 16.683775], 5);

L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
  attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

var wmsLayer = L.tileLayer.wms('http://wms3.nve.no/map/services/Nedborfelt1/MapServer/WmsServer?', {
  layers: ['Vassdragsomrade', 'Nedborfelt_til_hav'],
  transparent: true,
  format: "image/png",
  attribution: "NVE"
}).addTo(map);
<div id="map"></div>
body {
  margin: 0;
  padding: 0;
}

#map {
  position: absolute;
  top: 0;
  bottom: 0;
  right: 0;
  left: 0;
}

External resources loaded into this fiddle: