This plugin was extracted from Pavel Shramov's Leaflet Plugins repository in order to maintain this code more frequently and separate KML layer from other plugins.

So far we have fixed few issues.

Probably will work on Leaflet 1+, tested on Leaflet 1.4.

<html>
    <head>
        <link rel="stylesheet" href="http://unpkg.com/leaflet@1.4.0/dist/leaflet.css" />
        <script src="http://unpkg.com/leaflet@1.4.0/dist/leaflet.js"></script>
        <script src="./L.KML.js"></script>
    </head>
    <body>
        <div style="width: 100vw; height: 100vh" id="map"></div>
        <script type="text/javascript">
            // Make basemap
            const map = new L.Map('map', { center: new L.LatLng(58.4, 43.0), zoom: 11 });
            const osm = new L.TileLayer('http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png');
            map.addLayer(osm);
            // Load kml file
            fetch('assets/example1.kml')
                .then(res => res.text())
                .then(kmltext => {
                    // Create new kml overlay
                    const parser = new DOMParser();
                    const kml = parser.parseFromString(kmltext, 'text/xml');
                    const track = new L.KML(kml);
                    map.addLayer(track);
                    // Adjust map to show the kml
                    const bounds = track.getBounds();
                    map.fitBounds(bounds);
                });
        </script>
    </body>
</html>