Can I use
?

Online/offline status

- LS

Events to indicate when the user's connected (online and offline events) and the navigator.onLine property to see current status.

Chrome

  1. 4 - 13: Not supported
  2. 14 - 112: Supported
  3. 113: Supported
  4. 114 - 116: Supported

Edge

  1. 12 - 112: Supported
  2. 113: Supported

Safari

  1. 3.1 - 4: Not supported
  2. 5 - 16.4: Supported
  3. 16.5: Supported
  4. 16.6 - TP: Supported

Firefox

  1. 2 - 3: Not supported
  2. 3.5 - 3.6: Supported
  3. 4 - 40: Partial support
  4. 41 - 112: Supported
  5. 113: Supported
  6. 114 - 115: Supported

Opera

  1. 9 - 12: Not supported
  2. 12.1: Partial support
  3. 15 - 98: Supported
  4. 99: Supported

IE

  1. 5.5 - 7: Not supported
  2. 8: Partial support
  3. 9 - 10: Supported
  4. 11: Supported

Chrome for Android

  1. 113: Supported

Safari on iOS

  1. 3.2 - 4.1: Support unknown
  2. 4.2 - 16.4: Supported
  3. 16.5: Supported

Samsung Internet

  1. 4 - 20: Supported
  2. 21: Supported

Opera Mini

  1. all: Not supported

Opera Mobile

  1. 10 - 12.1: Not supported
  2. 73: Supported

UC Browser for Android

  1. 13.4: Supported

Android Browser

  1. 2.1 - 2.2: Support unknown
  2. 2.3 - 4.4.4: Supported
  3. 113: Supported

Firefox for Android

  1. 113: Supported

QQ Browser

  1. 13.1: Supported

Baidu Browser

  1. 13.18: Supported

KaiOS Browser

  1. 2.5: Supported
  2. 3: Supported

"online" does not always mean connection to the internet, it can also just mean connection to some network.

Early versions of Chrome and Safari always reported "true" for navigator.onLine

Resources:
MDN Web Docs - NavigatorOnLine.onLine