Just a short note: Based on the comments above, it should be possible to update that data set, to no longer use the KHR_techniques_webgl extension. For this, you can use the upgrade command of the 3d-tiles-tools.
Note: The current version is not yet released into npm, but should be released soon (it will probably be version 0.2.1).
To apply that command to your tileset, you can use
git clone https://github.com/CesiumGS/3d-tiles-tools.gitcd 3d-tiles-toolsnpm installnpx ts-node ./src/main.ts upgrade -i ./example/tileset.json -o ./example-upgraded/tileset.json