The BCD (Browser Compat Data) can be downloaded from the release area of the official MDN repository (https://github.com/mdn/browser-compat-data/releases, find data.json, about 12MB in size) or download from CDN sites like https://unpkg.com/@mdn/browser-compat-data/data.json
To update WebIDL files, you will need to have firefox source code avaiable (more specifically, its dom/webidl and dom/chrome-webidl subdirs)
# to download (from their git mirror)
git clone --depth=1 -b release https://github.com/mozilla/gecko-dev