Sketch 3.4 introduced the ability to securely share the contents of your file to a local server. This can be done by clicking the Share item in the toolbar.
Once clicked, a popover will appear asking you to flick the switch and share your content. If you decide to do this, a browser window will appear and your designs can now be previewed and any changes you make in Sketch will be reflected in the browser as the preview automatically updates.
This means that you can update, share, and preview your Artboards live on Windows, Android, iOS, or any other device with a browser — whether that be mobile, TV, or fridge — that is connected to the same local network.
Note: Once you share the contents of your document, it can be viewable by anyone on the local server. To stop sharing, simply turn the switch in the popover off.