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* They often call one or more servers for their data., e.g., Amazon, Google, LibraryThing, Open Library, WorldCat.
* The programs tend to be specific to a platform, e.g., WordPress, Drupal, Firefox.
Platforms such as WordPress and Drupal require the website author to install the widget; all users can see the widget. Platforms such as Firefox require the user to install the widget; only users who have installed the plugin can see the widget.
* Widgets are portable. They are easily embedded or modified in their target environment.