Fundamental to keyboard-only and screen reader users is navigational access and interaction using the keyboard alone. This includes means to set keyboard focus to toolstrips and Omnibox icons for background page functionality, as well as any popups, dialogs or other UI elements the extensions might generate. Also required is a way to discern and trigger any associated behavior/action connected to these UI elements.
Setting keyboard focus to a toolstrip could be provided either through a keyboard shortcut (much like we have for the toolbar), or by unifying access to all of our toolbars (toolbar, infobars, bookmarks, extensions) by a single shortcut and a way to circle through them. The first approach is simpler, but the second has more long-term merit, avoiding discoverability issues with multiple keyboard shortcuts.
Once focus is on the toolstrip, arrow keys should be used to traverse between toolstrip buttons, and Space/Enter keys to activate them.
The first piece for supporting screen readers is provided by enabling full keyboard access (including focus tracking) to the UI. Secondly, correct information needs to be exposed through the platform-specific accessibility interface (MSAA on Windows). Providing this information for toolstrips themselves is almost landed (http://codereview.chromium.org/155446), and the next step will be to provide coverage for the toolstrip buttons.
The real challenge here will be to appropriately give access to the information otherwise connected to the toolstrip buttons (e.g. the slideout with buildbot status), as well as interactive Omnibox icons.
The Extensions toolstrip already respects OS font resizing (on Windows: Control Panel > Display > Appearance tab, and choose 'Extra Large Fonts'). Theming (as below with High Contrast) will help for users with contrast issues.
All of the extensions UI should respect the theming imposed by the High Contrast mode. Support for this is being developed this quarter (Q3) by Stephen White, and should cover Extensions as well.
Anyone developing an extension should be aware of accessibility best practices for the parts not covered by Chrome browser accessibility. Included in this are things such as HTML files and JS content (including dynamically generated HTML).
Developer documentation pages should be built along accepted guidelines and best practices for accessible static HTML, where appropriate.