Leaflet with a basemap in EPSG:28992

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>A basic map with Leaflet</title>
    
    
     <link rel="stylesheet" href="https://unpkg.com/leaflet@1.3.1/dist/leaflet.css"
      integrity="sha512-Rksm5RenBEKSKFjgI3a41vrjkw4EVPlJ3+OiI65vTjIdo9brlAacEuKOiQ5OFh7cOI1bkDwLqdLw3Zg0cRJAAQ=="
      crossorigin=""/>
    
    
    <style type="text/css">
      body, html {
        height: 90%;
      }
      #map-container {
        height: 100%;
      }
    </style>
  </head>
  <body>
    
    
    <div id="map-container"></div>
    
    
    <script src="https://unpkg.com/leaflet@1.3.1/dist/leaflet.js"
    integrity="sha512-/Nsx9X4HebavoBvEBuyp3I7od5tA0UzAxs+j83KgC8PU0kgB4XiK4Lfe4y4cgBtaRJQEIFCW+oC506aPT2L1zw=="
    crossorigin=""></script>

<script src="https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.4.4/proj4.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/proj4leaflet/1.0.2/proj4leaflet.min.js"></script>
    
    
    <script>
      //define the custom projection tileset (Amersfoort/RD)
      var RD = new L.Proj.CRS( 'EPSG:28992','+proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +towgs84=565.2369,50.0087,465.658,-0.406857330322398,0.350732676542563,-1.8703473836068,4.0812 +no_defs', {
          resolutions: [3440.640, 1720.320, 860.160, 430.080, 215.040, 107.520, 53.760, 26.880, 13.440, 6.720, 3.360, 1.680, 0.840, 0.420, 0.210],
          bounds: L.bounds([-285401.92, 22598.08], [595401.9199999999, 903401.9199999999]),
          origin: [-285401.92, 22598.08]
        }
      );
      //initialize the map with our coordinate reference system
      let map = L.map('map-container', {
        crs: RD
      });
      //map view still gets set with Latitude/Longitude,
      //but the zoomlevel is now different (it uses the resolutions defined in our projection tileset above)
      map.setView([52.268, 4.998], 3)  ;

//add background layer in custom projection
      let bglayer_BRTAchtergrondkaart = new L.TileLayer('https://geodata.nationaalgeoregister.nl/tiles/service/tms/1.0.0/brtachtergrondkaartgrijs/EPSG:28992/{z}/{x}/{y}.png', {
        minZoom: 0,
        maxZoom: 13,
        tms: true,
        attribution: 'Map data: <a href="https://www.kadaster.nl">Kadaster</a>'
      });
      bglayer_BRTAchtergrondkaart.addTo(map) 
    </script>
  </body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.4.4/proj4.js

https://cdnjs.cloudflare.com/ajax/libs/proj4leaflet/1.0.2/proj4leaflet.min.js