Edit in JSFiddle 
<!-- Replace <YOUR_API_ACCESS_TOKEN> with your LocationIQ token before using this code. -->
<!DOCTYPE html>
<html>
    <head>
        <meta charset='utf-8' />
        <title>Add a marker</title>
        <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
        
        <script src='https://tiles.locationiq.com/v3/libs/maplibre-gl/5.24.0/maplibre-gl.js?v=0.1.9'></script>
        <link href='https://tiles.locationiq.com/v3/libs/maplibre-gl/5.24.0/maplibre-gl.css?v=0.1.9' rel='stylesheet' />
        
        <script src='https://tiles.locationiq.com/v3/libs/liq/liq-styles-ctrl-libre-gl-v5.js?v=0.1.9'></script>
        <link href='https://tiles.locationiq.com/v3/css/liq-styles-ctrl-libre-gl.css?v=0.1.9' rel="stylesheet" />
        
        <style>
            body { margin:0px; padding:0px; }
            #map { position:absolute; top:0px; bottom:0px; width:100%; }
        </style>
    </head>
    
    <body>
        <style>
            /*Example 1, all the CSS is defined here and not in JS*/            
            #markerWithExternalCss {
                background-image: url(https://tiles.locationiq.com/static/images/marker.png);
                background-size: cover;
                width: 100px;
                height: 100px;
                cursor: pointer;
            }
            
            /*Example 2, most of the CSS is set by JS*/
            .marker {
                display: block;
                border: none;
                cursor: pointer;
                padding: 0;
            }

        </style>
        
        <div id='map'></div>
        <script>
            
            //Add your LocationIQ Access Token here - https://my.locationiq.com/
            locationiq.key = '<YOUR_API_ACCESS_TOKEN>';
            //Define the map and configure the map's theme
            var map = new maplibregl.Map({
                container: 'map',
                style: locationiq.getLayer("Streets"),
                zoom: 12,
                center: [-122.42, 37.779]
            });
                        
            //Marker can be style either while adding the marker using JS or separately using CSS

            //Here's an example where we use external CSS to specify background image, size, etc
            //See the MapLibre GL Marker API for more options.
            // first create DOM element for the marker
            var el = document.createElement('div');
            el.id = 'markerWithExternalCss';
            // finally, create the marker
            var markerWithExternalCss = new maplibregl.Marker({ element: el })
                .setLngLat([-122.444733, 37.767443])
                .addTo(map);

            //Here's an example where use set variables like backgroundImage in JS itself (the common params are specified in CSS '.marker'
            var el2 = document.createElement('div');
            el2.className = 'marker';
            el2.style.backgroundImage = 'url(https://tiles.locationiq.com/static/images/marker50px.png)';
            el2.style.width = '50px';
            el2.style.height = '50px';
            
            // add marker to map
            new maplibregl.Marker({ element: el2 })
                .setLngLat([-122.4727000, 37.786258])
                .addTo(map);

        </script>
    </body>
</html>