You can always use the Anima toolkit to revise your Sketch file locally without an internet connection. Offline edits to your design will only apply to your Sketch file.
Sync / Preview in Browser / Export Code
When it's time to export HTML or preview your website, designs we translate designs to web using an online conversion. You'll need an internet connection to complete this process.
If your organization uses a secure firewall, you may want to consider adding these exemptions for Anima to function as intended: