"BeforeUnloadEvent" user message presets

[delta-9]

https://html.spec.whatwg.org/multipage/browsing-the-web.html#the-beforeunloadevent-interface:beforeunloadevent

BeforeUnloadEvent

I'm sure it's not the right place to file this issue.

A possible middle ground between custom message prohibition and allowing them would be to create a preset of custom message identified by a constant.

Those message would be crafted ahead to match the common use case where that warning is intended for good purposes, each would be made so they could not be used to deceive the user.

In some use cases "Changes may not be saved" or "This page is asking you to confirm that you want to leave — information you’ve entered may not be saved." works, but it would be great if we could give a better reason to the user.

To give an idea:
UNSAVED_CHANGES = "Your changes may not be saved"
PAYMENT_PROCESSING = "Your payment is still processing"
UPLOADS_PROCESSING = "File uploads are still processing and could be lost"
FORM_DATA_PROCESSING = "The information you entered may not be saved"
ETC..

interface BeforeUnloadEvent : [Event](https://dom.spec.whatwg.org/#interface-event) {
  attribute DOMString ['custom value', 'UNSAVED_CHANGES', 'PAYMENT_PROCESSING', 'UPLOADS_PROCESSING', 'FORM_DATA_PROCESSING', '...'];
};

Maybe I'm missing something, also please redirect me to the right place to post this feature request.