The application should be as usable by as many people as possible. For more, read the accessibility page in the Bitcoin Design Guide, and the Wikipedia entry on web accessibility.
A few things to consider:
- Clear accessibility labels of UI elements (most applications get this wrong)
- Visual highlights for focused state of UI elements
- Multi-sensory feedback on interactions
- Alternate interaction options
- Sufficient visual contrast
- Support dynamic text size
- Simple, clear language
- Keyboard navigation
- Voice over support
Operating systems allow users to personalize display settings. Options include increased contrast, bold text, larger or smaller text sizes, and much more. These should be supported as much as possible.