Alternative keyboards cover a wide range of hardware built for people who cannot use a standard keyboard comfortably or at all: large-key keyboards for users with limited fine motor control, one-handed keyboards, compact keyboards for users with limited reach, and keyguards that fit over a standard keyboard to prevent accidental key presses.
What matters for web content is the same as any keyboard: the page must be fully operable using standard keyboard commands, Tab, Shift+Tab, Enter, arrow keys, since alternative keyboards send the same standard key signals to the operating system that a conventional keyboard does.