I’ve played today with this feature of HTML 5. If your browser supports it, using this, everyone can find out your exact location (it really is pretty accurate!). You will have to permit the browser to share your location.
Here’s the app I’ve made.
Even if it sounds a bit spooky, knowing where your site’s viewer are from may come in handy in various situations. And this method it more accurate than using the user’s IP address. This API uses Global Positioning System (GPS) and location inferred from network signals such as IP address, RFID, WiFi and Bluetooth MAC addresses, and GSM/CDMA cell IDs to find the location.
You can read the api specification here.