Chrome for Android has long had support for installable Web apps via the "WebApp manifest" that developers can add to their pages. Actually installing a Web app has always been tougher than necessary, though, since the feature is buried in the Chrome menu as the "add to home screen" item. The Chrome team's solution has been to create an "App Install Banner," a site-triggered popup that prompts the user to add a page to his or her home screen. The feature is going live in Chrome 42, currently in beta.
To stop the feature from driving users insane, the popup will only show if you are a repeat visitor to the site. The requirements are listed as "The user has visited your site twice over two separate days during the course of two weeks."
It's Chrome that makes this determination, which should stop developers from gaming the system. If you dismiss the popup, it won't show again unless you clear your history. Google wants to strike a balance between informing users of the feature when relevant and not annoying them with home screen popups. "We will likely be changing all the heuristics over time," the company notes.
Web Apps aren't just bookmarks that go on your home screen; the project hopes to bridge the gap between webpages and installed apps with several extra features. Adding a site to your home screen is called "installing" because it also grants the site more access to your device. Sites that use the WebApp manifest can specify full-screen or Chromeless mode and set the screen orientation. Most importantly, they can register Service Workers, which can store data for offline usage and perform background tasks like syncing and push notifications.
The new popup isn't going to work on every site. Chrome's implementation will only work for sites that offer an app-like experience. Google requires that sites serve themselves in HTTPS, have a service worker on the site, and use the WebApp manifest.
There's no word yet on if users can turn it off.
Correction: Adding a site to your home screen doesn't grant it more permissions. APIs like Service Workers and view settings are available to developers regardless of whether the user puts an icon on the home screen. Some functions of Service Workers—like push notifications and running in the background—require user consent, which is asked for in a separate piece of UI.