There are two main issues to think about when writing new native Mac code: full keyboard access and VoiceOver compatibility.
On Mac OS X, the operating system is not fully keyboard-accessible by default. For example, there's no way to access the menu bar or most toolbars without using the mouse. However, users can turn on Full Keyboard Access by pressing Control+F1, and they can separately control whether they want Tab to cycle between all controls or just text boxes and lists by pressing Control+F7, or in System Preferences > Keyboard > Keyboard Shortcuts. When Full Keyboard Access is enabled, users can press keys like Control+F2 to highlight the menu bar, Control+F3 to highlight the Dock, and Control+F5 to highlight an application's toolbar.
When Full Keyboard Access is enabled, Chromium should provide full keyboard access to all controls. In many cases, Cocoa will take care of this for you automatically, so you should simply test its behavior. If you're implementing a custom UI widget, you may need to implement it yourself. To determine if Full Keyboard Access is enabled programmatically, query isFullKeyboardAccessEnabled in NSApplication.
Mac OS X includes a built-in accessibility service called VoiceOver that provides an alternative experience for users who are blind or low-vision. VoiceOver works quite well with Chromium today and it's important that we keep it that way!
It can take some time to master VoiceOver, but the basics aren't too hard. Press Cmd+F5 to toggle it on or off; give it a few seconds to start up if you haven't used it before. Consider going through the tutorial.
VoiceOver provides a synthesized-speech interface by default, but users with a refreshable braille display can access equivalent content in braille form, too. If you're not used to listening to synthesized speech, you can turn down the volume and ignore it, and instead just watch the Caption Panel that shows you everything VoiceOver is saying.
When testing with VoiceOver, it's very important that you only use VoiceOver keys to interact. Don't test things using other keyboard shortcuts you might know, because you won't be testing whether or not your interface is discoverable for VoiceOver users.
The VoiceOver keys are Control+Option and are abbreviated VO. Keep your fingers on these keys! Here are the keys you need to get started:
NSString* description = l10n_util::GetNSStringWithFixup(IDS_MY_BUTTON_LABEL);
For detailed information, start here: Accessibility Programming Guidelines for Cocoa (developer.apple.com)