The background_color attribute in the JSON Web Application Manifest allows web developers to provide a hint to Chrome indicating the background color of their web application. This color is used as the background of the application while its resources are fetched.
Docs: https://developers.google.com/web/updates/2015/09/using-web-app-manifest-to-set-solid-color-loading-screen https://developer.mozilla.org/en-US/docs/Web/Manifest