Tl;dr: the presence of unload event listeners is a primary blocker for back/forward cache on Chromium based browsers and for Firefox on desktop platforms. On the other hand, for mobile platforms, almost all browsers prioritize the bfcache by not firing unload events in most cases. To improve the situation, we’ve been working with lots of partners and successfully reduced the use of unload event listeners over the last few years. To further accelerate this migration, we propose to have Chrome for desktop gradually skip unload events. If this call for feedback doesn’t unearth critical showstoppers and if the proposal makes it through the blink process, the behavior change could be starting from M114 at the earliest (note: beforeunload will remain unchanged). We’d like feedback on this plan, in particular use cases that don’t yet have a viable alternative.
This is a call for feedback about a tentative plan regarding unload events. Our goal is to identify use cases for which there isn’t any good alternative to unload events, and would therefore prevent this plan from moving forward.
The unload event is extremely unreliable. It is ignored in most cases by all mobile browsers with the exception of Firefox on Android. Furthermore, in Safari, the unload event is ignored on both desktop & mobile platforms. In addition to being unreliable, the presence of unload event listeners on a page is a major back/forward cache blocker on desktop for Chromium browsers and Firefox. Based on Chrome stats, we believe that unload event listeners reduce bfcache’s ability to deliver instant back/forward navigation by ~18 percentage points (hit-rate).
Over the course of 2021~2022, we ran a large collaborative effort to reduce the usage of unload event listeners, in particular across popular third parties. We’ve seen great progress with many sites and third parties having already completed their migration.
Given how unreliable unload events are, the potential user experience upsides, and the great progress achieved by the ecosystem on switching away from unload, we’d like to help accelerate the migration by gradually skipping unload events on Chrome for desktop.
👉 Please note that beforeunload will remain unchanged as this event doesn’t have reliability issues and doesn’t block BFCache. 👈
We are interested in hearing your feedback about this plan. In particular, please let us know if you are aware of unload event listener use cases that lack a viable alternative. Your feedback will inform the proposal (e.g. behavior and timeline).
If this call for feedback doesn’t unearth any critical showstoppers, and if the proposal makes it through the blink process, we’d start the plan from M114 at the earliest by having a small likelihood of ignoring unload events while providing access to fine-tuning control (e.g. Permission-Policy: unload API) and Enterprise/Edu carve outs. From there, we’ll continue to monitor the community’s feedback and gradually increase the likelihood over time. We are hoping to make significant progress by the end of this year, and hope to reach a satisfying state sometime in 2024.
See the sections below for more context, our guidance for a post-unload web, an API to exert control over unload event listeners, and our approach to ease-in enterprise/edu products into this change.
Background about bfcache
The multiple behaviors of bfcache with unload events
Unfortunately, not all pages can be stored in bfcache. For instance, using certain APIs prevent pages from entering the bfcache. In particular, the presence of unload listeners on a page is the most common bfcache blocker.
The use of unload listeners is highly discouraged because it’s a fundamentally unreliable event:
On desktop, Chrome and Firefox are currently firing unload events at the cost of the user experience, while Safari will attempt to cache some pages with an unload event listener (skipping the event in doing so).
On mobile, Chrome and Safari will attempt to cache pages with an unload event listener. On the other hand, Firefox treats pages that use unload event listeners as ineligible for the bfcache, except on iOS, which requires all browsers to use the WebKit rendering engine (i.e. all browsers inherently behave like Safari on this platform).
Alternatives to unload event listener
The recommended alternatives to unload event listeners are to:
Use the pagehide event listener (note: despite the name, this serves a different purpose than the page visibility API).
For the cases where user interaction would be useful, conditionally use the beforeunload event listener.
In addition, you may be interested in the origin trial for the Pending Beacon API. This bfcache-friendly API allows sending a bundle of data to a backend server, ideally at the ‘end’ of a user’s visit to a page. From our observations, we believe this is the most common use case for unload event listeners. Compared to the methods highlighted above, this API has better ergonomics.
Test driving a web free of unload event listeners!
Exert control over unload event listeners (e.g. completely disallow them, or selectively allow them for specific origins).
Report the use of unload event listeners to an endpoint for assessment purposes.
Chrome for Enterprise & Education
We also acknowledge that providers of enterprise & education solutions may not always have the flexibility to quickly update existing deployments. To minimize concerns, we’ll offer a group policy to keep the current behavior for unload events. This policy will also be enabled by default if Chrome detects that it is in an enterprise / education environment, as hinted by the presence of one or more existing group policies. For unmanaged Enterprise/Edu environments, a simple Chrome extension could inject the relevant calls to the Permission-Policy:unload API for temporarily opting-out the relevant origin(s).
Will there be a way to detect whether or not unload will fire from JS?
We (Akamai) have some security products that may hook into unload, only if they detect that another script has hooked into unload. When this happens, we also shift some "cleanup" work to that unload handler that we need to execute. Otherwise the cleanup work executes in beforeunload/pagehide/etc.
If we have knowledge that unload won't execute, we would skip hooking unload and keep our cleanup behavior earlier. (we don't hook unload for any other reason).
For browsers that don't reliably execute unload today (e.g. Safari), we a combination of UA sniffing and feature detection. But that could be tricky if Chrome transitions from reliably firing it to not over time.
Will there be a way to detect whether or not unload will fire from JS?I don't believe there is a way to query the policies that currently apply to a document.Ian, is there? Should there be?