Here I want to have simplest example of google maps (in order to be able to copy paste it :) )
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <script defer type="text/javascript" src="https://maps.googleapis.com/maps/api/js?key=yourKey&callback=initMap"></script>
    <title></title>
</head>
<body>

    <div id="map-canvas"></div>

    <script>
        function initMap() {
            var mapCanvas = document.getElementById('map-canvas');
            var mapOptions = {
                zoom: 6,
                center: { lat: -34.397, lng: 150.644},
                mapTypeId: google.maps.MapTypeId.ROADMAP
            };

            map = new google.maps.Map(mapCanvas, mapOptions);
        }
    </script>

<style>
    html, body {
        width: 100%;
        height: 100%;
        margin: 0;
        padding: 0;
    }

    #map-canvas {
        width: 100%; 
        height: 100%; 
    }
</style>

</body>
</html>