This feature enables progressive web apps to register as https URL handlers using a new member in the web app manifest. When the browser is started by a URL launch from the OS, it will look for matching URL handlers and prompt the user to launch the URL in a web app. Where possible, web apps will be registered directly with the host OS as a URL handler for a more native experience.
This feature is new to the web and web apps that do not opt in will not be affected because any additions to the web app manifest will be optional. Browser behavior will also not be affected if there are no installed apps that opt in. Browser that do not support this feature will not be adversely affected by new members in the web app manifest.
Gecko: No signal
WebKit: No signal
Web developers: No signals
Debug functionality will be added the the DevTools(F12) Application pane.
Yes. It will be supported in the browser on all platforms. Integration with OS features for native URL handling might not be possible on all platforms.
No. Web app manifest changes that are part of this feature can be tested in WPT app manifest tests: https://github.com/web-platform-tests/wpt/tree/master/appmanifest