Edit

Example of a Stamen tile source.

Two layers are composed: the watercolor base layer with the terrain labels.

main.js
import 'ol/ol.css';
import Map from 'ol/Map';
import Stamen from 'ol/source/Stamen';
import TileLayer from 'ol/layer/Tile';
import View from 'ol/View';
import {fromLonLat} from 'ol/proj';

const map = new Map({
  layers: [
    new TileLayer({
      source: new Stamen({
        layer: 'watercolor',
      }),
    }),
    new TileLayer({
      source: new Stamen({
        layer: 'terrain-labels',
      }),
    }),
  ],
  target: 'map',
  view: new View({
    center: fromLonLat([-122.416667, 37.783333]),
    zoom: 12,
  }),
});
index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Stamen Tiles</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep"></script>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,URL,TextDecoder,Number.isInteger"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script src="main.js"></script>
  </body>
</html>
package.json
{
  "name": "stamen",
  "dependencies": {
    "ol": "6.8.1"
  },
  "devDependencies": {
    "parcel": "^2.0.0-beta.1"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --public-url . index.html"
  }
}