I'm guessing they mean some kind of situation where the resources you're zipping up have already been downloaded to the user's browser (like a "download as Zip"

I'm guessing they mean some kind of situation where the resources you're zipping up have already been downloaded to the user's browser (like a "download as Zip" link for a set of images that are being displayed on the page). In reality, I don't think there is a good use-case for this as any files that need to be zipped are likely too large for a user's browser to process with JS (it could bog down or crash the browser). I have a feeling they already realize this though as the example zip file is 237 bytes. If an HTTP header is larger than your zipped file, it probably doesn't need to be zipped/gzipped in the first place.