Using White Spaces to work offline is as easy as opening a script and selecting 'Make Available Offline' in the 'File' menu on the left-hand side.
Once selected a loading indicator will be displayed - this is just showing that we are syncing your script to the browser. Don't close the page or navigate away whilst the indicator is displaying as it may not complete and you could have a partial download. When it's successfully downloaded it'll look like this:
To remove the downloaded version, just select 'Remove Offline Data'. Go ahead, turn off your internet and watch it load as if you were connected!