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Accepted. No NPAPI version number.

Problem Summary

Plugins cannot currently provide accessible information about their contents on Linux platforms. This is Mozilla bug 78414.


  • Accessibility - the degree to which a product is accessible by as many people as possible.
  • Assistive Technology (AT) - a piece of software or hardware that connects to the system's accessibility services to provide assistance to the user. This assistance can come in the form of a head tracking system for a user unable to use a mouse, a screen reader for a person with vision impairments, etc.
  • Microsoft Active Accessibility (MSAA) - the standard accessibility API for Windows 95 through Windows XP. Accessible objects in MSAA inherit from the IAccessible interface.
  • Accessibility Toolkit (ATK) - the standard accessibility API on Linux.
  • Document Object Model (DOM) - a in-memory tree representation of the content of a web page.
  • Interprocess Communication (IPC) - a way of communicating betweenthread and process boundaries.
  • Plugin Host - in an out of process situation, the code which bootstraps the plugin inside of a separate process than the browser. In an in process situation, this code resides in the main process.

Related Approaches

On Windows, plugins can provide their own accessibility support by responding to the WM_GETOBJECT WinEvent and returning an instance of IAccessible. Firefox (and other browsers) will call this WinEvent on the plugin object and if an IAccessible is returned, register the accessible into the accessibility hierarchy.

API Requirements

  • Plugins should be able to easily provide an accessible implementation (in ATK) to the Browser
  • The Browser should inject the plugin's accessible into its accessible hierarchy
  • The lifetime of the plugin's accessible should be the same as the lifetime of the plugin.
  • Browsers should be able to implement this specification for both in-process as well as out-of-process plugins.

Current Proposal


NPAPI Native Accessibility Support for Linux

  • Last modified: Apr 28, 2010
  • Author: Brad Taylor, Novell, Inc.
  • Contributors:

Plugins have the need to provide accessibility support to the browser so they can be accessed by assistive technologies like screen readers and testing tools. Most common web browsers already provide an accessible implementation of DOM content, but thus far, can only export a blank (and therefore useless) object when it comes across a plugin.

NOTE: This approach should be extensible to other accessibility frameworks, e.g.: windowless Windows plugins or OS X Accessibility.

To allow plugins to provide their own accessibility support in both in process (IP) and out of process (OOP) scenarios, we leverage the AT-SPI registry to coalesce accessibiles from the browser and the plugin. Two corresponding objects are involved in this process in ATK, namely AtkPlug and AtkSocket. Together, they are used to connect accessibility hierarchies in a manner that is transparent to ATs. These APIs work both for accessibles in the same process, as well as accessibles in different processes.

In this example, the browser would create an AtkSocket object, and place it in the plugin's spot on the accessibility hierarchy. The plugin's host would then call NPP_GetValue with NPPVpluginNativeAccessibleAtkPlugId (with value 19) as the variable parameter.

Compatible plugins would return the value of atk_plug_get_id() (a char*), or NULL indicating that they do not provide remote accessibility support.

A valid return value would be remoted (in an OOP scenario), or returned to the browser (in an IP scheme), and used to attach the plug to the socket via atk_socket_embed(). The connection that is established via AT-SPI and atk-bridge in the background should be not only transparent to ATs, but be seamless for the browser.

Vendors are expected to use g_object_ref and g_object_unref as normal to manage the lifecycle of plugs and sockets.

A detailed description of the mechanics of this API in the Linux accessibility stack is available on the gnome-accessibility-devel mailing list.

The new AtkPlug and AtkSocket API is now available in ATK 1.30 and AT-SPI2 0.1.3. It will be shipped as the default accessibility toolkit by distributions which ship GNOME 3.0.