This example uses OpenLayers v10.9.0. The latest is v.
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#### Semi-Transparent Layer

transparent 1 osm 6 tilejson 5 css 2

#####

Example of a map with a semi-transparent layer.

This example will display a tiled MaxBox layer semi-transparently over an OSM background. The OSM layer is changed to black and white by using a CSS filter.

##### main.js

    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import TileLayer from 'ol/layer/Tile.js';
    import {fromLonLat} from 'ol/proj.js';
    import OSM from 'ol/source/OSM.js';
    import TileJSON from 'ol/source/TileJSON.js';

    const key = 'Your Mapbox access token from https://mapbox.com/ here';

    const map = new Map({
      layers: [
        new TileLayer({
          className: 'bw',
          source: new OSM(),
        }),
        new TileLayer({
          source: new TileJSON({
            url:
              'https://api.tiles.mapbox.com/v4/mapbox.va-quake-aug.json?secure&access_token=' +
              key,
            crossOrigin: 'anonymous',
            // this layer has transparency, so do not fade tiles:
            transition: 0,
          }),
        }),
      ],
      target: 'map',
      view: new View({
        center: fromLonLat([-77.93255, 37.9555]),
        zoom: 7,
      }),
    });

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>Semi-Transparent Layer</title>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <style>
          .map {
            width: 100%;
            height: 400px;
          }
          .bw {
            filter: grayscale(100%);
          }
        </style>
      </head>
      <body>
        <div id="map" class="map"></div>

        <script type="module" src="main.js"></script>
      </body>
    </html>

##### package.json

    {
      "name": "semi-transparent-layer",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }