WMS 1.3.0 BBOX is lat-first, so the layer is offset

• VERSION=1.1.1&SRS=EPSG:4326&BBOX=116.0,39.7,116.6,40.1 returns the correct Beijing image.
• The same request as VERSION=1.3.0&CRS=EPSG:4326 returns a 256x256 pure white tile.
• Reordering to BBOX=39.7,116.0,40.1,116.6 restores the image but it is upside down.

1. Read VERSION from the request URL; 1.1.1 and 1.3.0 have different axis rules.
2. Take absolute values of the four BBOX numbers; a value above 90 in the third slot means latitude is in the longitude position.
3. Query GetCapabilities and check whether the layer advertises CRS:84, which keeps lon-lat order even in 1.3.0.
4. Open the GetMap URL directly in the browser to prove the problem is the request, not the layer.
5. Let the library build the BBOX instead of hand-concatenating it.

Use L.tileLayer.wms or ol.source.TileWMS, which emit the correct axis order, or switch to CRS=CRS:84 when the server supports it; for hand-built URLs, wrap BBOX assembly in one function that branches on VERSION.

...&VERSION=1.1.1&SRS=EPSG:4326&BBOX=116.0,39.7,116.6,40.1
...&VERSION=1.3.0&CRS=EPSG:4326&BBOX=39.7,116.0,40.1,116.6
...&VERSION=1.3.0&CRS=CRS:84&BBOX=116.0,39.7,116.6,40.1

L.tileLayer.wms('http://localhost:8080/geoserver/gis/wms', {
  layers: 'gis:beijing_districts',
  format: 'image/png',
  transparent: true,
  version: '1.3.0',
  crs: L.CRS.EPSG4326
}).addTo(map);
new TileWMS({
  url: 'http://localhost:8080/geoserver/gis/wms',
  params: { LAYERS: 'gis:beijing_districts', TILED: true },
  serverType: 'geoserver',
  projection: 'EPSG:4326'
});

• Assuming 1.3.0 is just 1.1.1 with SRS renamed to CRS.
• Swapping the BBOX but leaving the image flipped because the CRS is still EPSG:4326 instead of CRS:84.