Work Offline (Page Only) от emano waldeck

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Deference from the built-in "work offline": this extension only blocks web pages from accessing the internet while the built-in "work offline" prevents the entire browser from assessing the web. So if you want to test a page locally and still get notifications from your add-ons, this extension could help
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This extension brings the "Work Offline" button to your browser. When the extension is in the offline mode, no web request can be sent or received by web pages. Note that still, your extension can use the internet. So let's say you have a web-based email client and an email notifier extension. The web-based one is disconnected, but the extension one is connected.

Notes:
1. You can enable and disable the offline mode right from the toolbar button. This is a toggle button.
2. If you have open tabs, those tabs cannot send and receive network requests in the offline mode
3. You can still browse local HTML files while you are in the offline mode, though these pages cannot anything to any web servers
4. This extension dispatches "online" and "offline" events when the status changes and also set "navigator.onLine" property accordingly.
5. As of version 0.1.4, this extension blocks both HTTP and WebSocket traffics on the offline mode