Edit in JSFiddle 
<!DOCTYPE html>
<html>
    <head>
        <meta charset='utf-8' />
        <title>Add multiple markers</title>
        <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
        
        <script src='https://tiles.locationiq.com/v3/libs/maplibre-gl/1.15.2/maplibre-gl.js'></script>
        <link href='https://tiles.locationiq.com/v3/libs/maplibre-gl/1.15.2/maplibre-gl.css' rel='stylesheet' />
        
        <script src='https://tiles.locationiq.com/v3/js/liq-styles-ctrl-libre-gl.js?v=0.1.8'></script>
        <link href='https://tiles.locationiq.com/v3/css/liq-styles-ctrl-libre-gl.css?v=0.1.8' rel="stylesheet" />
        
        <style>
            body { margin:0px; padding:0px; }
            #map { position:absolute; top:0px; bottom:0px; width:100%; }
        </style>
    </head>

    <body>
        <style>
            .marker {
                display: block;
                border: none;
                border-radius: 50%;
                cursor: pointer;
                padding: 0;
            }
            
        </style>
        <div id='map'></div>
        <script>
            
            //Add your LocationIQ Maps Access Token here (not the API token!)
            locationiq.key = 'pk.aa7f5d0539c5675b7f3429402939d8fa';
            //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]
            });

            //Add markers from geojson. This list can be generated dynamically with an AJAX call as well.
            var geojson = {
                "type": "FeatureCollection",
                "features": [
                    {
                        "type": "Feature",
                        "properties": {
                            "message": "Foo",
                            "iconSize": [60, 60]
                        },
                        "geometry": {
                            "type": "Point",
                            "coordinates": [
                                -122.421953,
                                37.764966
                            ]
                        }
                    },
                    {
                        "type": "Feature",
                        "properties": {
                            "message": "Bar",
                            "iconSize": [50, 50]
                        },
                        "geometry": {
                            "type": "Point",
                            "coordinates": [
                                -122.464677,
                                37.777209
                            ]
                        }
                    },
                    {
                        "type": "Feature",
                        "properties": {
                            "message": "Baz",
                            "iconSize": [40, 40]
                        },
                        "geometry": {
                            "type": "Point",
                            "coordinates": [
                                -122.484948,
                                37.780090
                            ]
                        }
                    }
                ]
            };

            //Add markers to map
            //https://www.mapbox.com/mapbox-gl-js/api#marker
            geojson.features.forEach(function(marker) {
                // create a DOM element for the marker
                var el = document.createElement('div');
                el.className = 'marker';
                el.style.backgroundImage = 'url(https://tiles.locationiq.com/static/images/marker50px.png)';
                el.style.width = '50px';
                el.style.height = '50px';

                //Instead of this click listener, we can attach a popup / infowindow to this marker (see next section)
                el.addEventListener('click', function() {
                    window.alert(marker.properties.message);
                });

                // add marker to map
                new maplibregl.Marker(el)
                    .setLngLat(marker.geometry.coordinates)
                    .addTo(map);
            });

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