JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://openlayers.org/en/v3.20.0/build/ol.js"></script>
<link rel="stylesheet" href="https://openlayers.org/en/v3.20.0/css/ol.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/FileSaver.js/1.3.3/FileSaver.min.js"></script>
<div id="map" class="map"></div>
<a id="export-png" class="btn btn-default"><i class="fa fa-download"></i> Download PNG</a>

JavaScript

// this example uses FileSaver.js for which we don't have an externs file.
var map = new ol.Map({
  layers: [
    new ol.layer.Tile({
      source: new ol.source.OSM()
    }),
    new ol.layer.Vector({
      source: new ol.source.Vector({
        url: 'https://openlayers.org/en/v3.20.0/examples/data/geojson/countries.geojson',
        format: new ol.format.GeoJSON()
      })
    })
  ],
  target: 'map',
  controls: ol.control.defaults({
    attributionOptions: /** @type {olx.control.AttributionOptions} */ ({
      collapsible: false
    })
  }),
  view: new ol.View({
    center: [0, 0],
    zoom: 2
  })
});

document.getElementById('export-png').addEventListener('click', function() {
  map.once('postcompose', function(event) {
    var canvas = event.context.canvas;
    event.context.textAlign = 'right';
    event.context.fillText('© My Copyright Text', canvas.width - 5, canvas.height - 5);
    canvas.toBlob(function(blob) {
      saveAs(blob, 'map.png');
    });
  });
  map.renderSync();
});