If you're working on WebKit code, here are some things to know about accessibility.
In WebCore, accessibility is off by default. When an operating system's native accessibility services access web content, WebCore is switched into accessibility mode, which creates a cache of a tree of AccessibilityObjects. For the most part, the tree of AccessibilityObjects mirrors the render tree, but there are some exceptions.
WebCore/accessibility/ contains a generic implementation of accessibility, and then the mac/, win/, etc. directories contain platform-specific subclasses or wrappers for this - classes that implement the platform accessibility API for each node in the accessible tree. However, Chromium doesn't use any of these platform-specific ports - instead Chromium uses a thin wrapper, defined in WebKit/chromium/public/WebAccessibilityObject.h, that shuttles the information to the browser process. The accessibility APIs are implemented in Chromium's main browser process.
When adding implementations of new HTML tags, make sure they're handled properly in WebCore/accessibility - is a new subclass needed? Is there a role that covers this element?
When adding new features to the shadow DOM of an existing control, make sure the shadow DOM nodes have proper accessibility annotations, like labels or titles. Make sure anything you add is keyboard-accessible.