Microsoft Windows is the most widely used desktop operating system in the world, and while there are some basic built-in features for users with disabilities, there's a much larger and richer ecosystem of third-party products that provide alternative ways to interact with Windows.
Windows has a tradition of being fully keyboard-accessible without any extra configuration needed, and Chromium should be no exception. There shouldn't be anything you can do with a mouse that you can't also do with the keyboard. Where possible, Chromium should use standard discoverable keyboard shortcuts, but where that's impossible or difficult, custom keyboard shortcuts are fine.
Most users are familiar with using Tab to cycle between focusable controls. Here are some other keys that are widely supported in Windows applications (and should be working great in Chromium, too):
Windows has a special high contrast mode intended for people with low vision. The most common variation is white-on-black text instead of black on white. While Mac OS X and Linux provide this at the OS compositor layer (so apps don't have to do anything), Windows implements this by changing system colors and expects apps to use these colors.
To try High Contrast mode, press Alt+Shift+PrintScreen. (Note: must be the left alt and left shift key.)
All UI drawn by Chromium that's outside of the webpage should either be 100% themable, or it should respect the system colors. So when choosing colors for text and backgrounds in particular, be sure to use system colors, not hardcoded colors. See GetSysSkColor in ui/gfx/color_utils.h as a starting point.
On Windows, users who have high-contrast mode enabled will see a bubble the first time they open Chrome, prompting them to install the High Contrast extension and a white-on-black theme from the web store (both optional). The High Contrast extension improves the readability of web content, and the themes improve the look of the rest of Chrome.
Microsoft Active Accessibility (MSAA) is a COM interface that Windows programs can implement in order to allow access to their interface from assistive technology such as screen readers, magnifiers, voice-control applications, and more. Chromium has good support for MSAA now and it's important to keep it that way! When writing native Windows UI code, think about whether you might need to expose any additional information to make it accessible.
To do a quick accessibility check, download AViewer and press F6 to track focus. Then switch to Chromium and use the keyboard to navigate any area of the UI you want to test. Then check the Role, Name, Value, and other properties in AViewer and make sure they're correct.
Note: Chrome doesn't enable accessibility for the main web area by default unless it detects a screen reader or other advanced assistive technology. To override this, run chrome.exe with the --force-renderer-accessibility flag.
To do a more involved check, learn to use NVDA, an open-source screen reader. Google employees also have access to site licenses for some of the most popular commercial tools, including JAWS and ZoomText.
Most of the time you probably won't be writing native Windows code, you'll be writing Views code. Views has its own accessibility layer, which should make things very easy - in most cases it will take only 2 or 3 lines of additional code to make a new control accessible. See Views Accessibility for more information.
If you are writing native Windows code, you may be able to update the accessibility information for a particular widget using dynamic annotation - that's the easiest. Here's an example that sets the accessible name of hwnd to name:
HRESULT hr = CoCreateInstance(CLSID_AccPropServices, NULL, CLSCTX_SERVER,
hr = acc_prop_services->SetHwndPropStr(hwnd, OBJID_CLIENT,
CHILDID_SELF, PROPID_ACC_NAME, name.c_str());
If all else fails, you'll have to implement a custom IAccessible interface for your widget. Hopefully not! So far Chrome doesn't have any examples of this, everything is either handled by the Views accessibility code in ui/views/accessibility, the web content accessibility code in content/browser/accessibility, or a few tiny annotations like the example above.