Web applications are simply Web sites with an accompanying configuration file. This file, the manifest, contains extra information necessary to install the Web app, which in some instances may make it available when there's no network connection.
Google's Web app specification makes a distinction between installable Web apps and hosted Web apps. The former rely on Google Chrome Extension APIs and only run in the Chrome browser. The latter are simply what we know today as Web sites and they can be accessed by typing the appropriate URL into one's Web browser.
These two approaches are not quite compatible, though efforts are being made to make them more so. Google Chrome Web apps are only available from the Chrome Web Store and can only be installed in the Chrome browser. Mozilla Open Web apps will be available from anyone who bothers to set up a Web store using Mozilla's specifications and can be installed in any compatible browser.
The concept of installation is mainly relevant in the context of published or installed Web apps that work when the user is offline. Installing a hosted or bookmarked Web app is not really any different than bookmarking a Web site today. However, as Web apps integrate new HTML5 technologies like geolocation, installation will provide a way to store preferences. This might make these Web apps more usable by requiring fewer permission request interruptions.
Chrome Web apps and Mozilla Web apps have minor differences in terms of the data stored in their respective manifest files. Chrome manifest files rely on absolute URL paths while Mozilla manifest files rely on relative URL paths, for example. Also, Mozilla manifests have an installs_allowed_from parameter while Chrome manifests have unique fields like container, key, and update_url.
In a blog post last fall, Ben Francis, a software engineer at Rabbitsoft, observed that his initial dubiousness about the need for installable Web apps has faded as the model begins to make more sense. He sees installable Web apps as a way to bring the distribution and discovery benefits of the app store model to cloud computing.
"This new breed of apps could leverage Web technologies to reach the whole range of competing platforms with a single application which can be distributed via a range of competing app stores," he wrote. "By 'installing' a Web application the user can pre-approve access to local hardware like offline storage, accelerated graphics and geolocation to allow the app to make the most of the hardware it's running on and the user can keep a local repository of all the applications they use."
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