Guide
Why the ETA screen has to stay open
ETA is a web app, and web apps play by the browser’s rules. Here’s what those rules are, where they come from, and what we do about them.
Because of these limits, never rely on ETA for anything where a missed alert could put someone at risk. In an emergency, call 999.
Location only reaches pages you can see
The W3C Geolocation specification, which browsers follow, says that if a page is hidden the browser waits until it’s visible again before getting a position, and that position updates are “exclusively for fully-active visible documents”.
Source: W3C Geolocation, section 6.5. Checked 20 October 2026.
A page counts as hidden when its tab is in the background, its window is minimised, or the phone’s screen is off (that definition is from MDN’s Page Visibility guide). MDN also notes that browsers slow down timers in background tabs to save battery. So while ETA is hidden it can’t see you moving, and can’t send an alert.
Keeping the screen on: the wake lock
To stop your phone dimming and locking during a journey, ETA uses the Screen Wake Lock API.
- It only works on secure (https) pages, and only while the page is visible.
- The browser drops the lock as soon as the page is hidden — so ETA asks again when you come back.
- The request can be refused, for example in power-saving mode or when the battery is low.
Source: MDN — Screen Wake Lock API. Checked 20 October 2026.
| Browser | Screen wake lock |
|---|---|
| Chrome (Android and desktop) | From version 84 |
| Samsung Internet | From version 14 |
| Firefox (Android and desktop) | From version 126 |
| Safari on iPhone and iPad | From 16.4 in the browser; in apps added to the Home Screen only from 18.4 (WebKit bug 254545) |
| Safari on Mac | From 16.4 |
Source: MDN — WakeLock browser compatibility. Checked 20 October 2026.
The journey screen shows “Screen on: yes, kept on” when the lock is working. If it says “not supported” or “refused”, change your phone’s auto-lock setting for the journey, or tap the screen now and then.
Sound needs a tap first
Browsers have autoplay rules: a page can only start making sound in response to you interacting with it (MDN’s autoplay guide). That’s why ETA prepares the alarm sound when you press Start or “Test the alarm”. Use the test button to check your volume before you set off.
Why not a notification?
Notifications from websites behave very differently between phones. On iPhone, according to MDN’s compatibility data, they’re only available to web apps added to the Home Screen. We haven’t built notifications into this first version, and a notification still wouldn’t solve the main problem: the page has to be visible to get your location.
Phase 2: a native app
Reliable alerts with the screen off, or with ETA in the background, would need a proper iPhone and Android app with background location permission. That’s on our list. Until then, the honest answer is: keep the journey screen open, plug in if you can, and keep the phone where you can hear it.
Checklist
- Open ETA in your phone’s browser (or from the Home Screen icon).
- Press “Test the alarm” and turn the volume up.
- Start the journey and check the status chips say GPS and “Screen on”.
- Don’t switch apps or lock the phone until you’ve arrived.
Facts on this page checked against the W3C Geolocation specification, MDN and MDN’s browser compatibility data on 20 October 2026.