Q:

OpenLayers tutorial: map doesn't show up

I'm following an OpenLayers tutorial and already there is something wrong after trying to represent a simple map. 
I will show you the code I copied from the tutorial and when opening my HTML file with Firefox, it should show me a OSM map. However it doesn't do anything. I have tried to write "Test" into my html, and that worked, thus I guess the problem lies somewhere else. 
That's the HTML code: 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name = "viewport" content = "width=device-width, initial-scale=1.0">
    <title>Title</title>
    <link rel="stylesheet" href="./lib/OpenLayers/ol.css">
    <link rel="stylesheet" href="./style.css">
</head>
<body>
    <div id="js-map" class="map"> </div>
    <script src='./lib/OpenLayers/ol.js'> </script>
    <script src='./Main.js'> </script>
</body>
</html>

That's the js code: 
    window.onload = init;

    function init(){
        const map = new ol.Map({
        view: new ol.View({
            center: [8.25, 49.97],
            zoom: 11.1,
            maxZoom: 15,
            minZoom: 5
        }),
        layers: [
            new ol.layer.Tile({
                source: new ol.source.OSM()
            })
        ],
        target: 'js-map'
    })
}

I guess, there is a typo somewhere but I just can't see it. 
Another possible problem is that I'm using Ubuntu. Perhaps it has something to do with that? 
Here is also a screenshot from the project folder: 

A:

In OpenLayers 6 the map div must have a size, such as
<style>
  .map {
    height: 400px;
    width: 100%;
  }
</style>

in the quick start example https://openlayers.org/en/latest/doc/quickstart.html

