BIOS and UEFI are the lowest-level parts of software on the computer, the firmware that handles CPU features and operating system boot. Even this is almost always inaccessible. Nothing can be done for BIOS systems, but UEFI should be extended to provide sound, and thus blind-accessibility, functions.
The prime directive is to communicate with users. The user will generally know what they would like rather than what wouldn’t help or even hinder their experience. Some users may have different ideas of what is “accessible” versus other users. In cases like this, you’ll either need to choose between the two ideas, or give the users a choice between the two (preferable). When it comes to accessibility, choice is never a bad thing. Your users will thank you for giving them something that makes their own experience enjoyable, productive, or both.
This section will concern users who have either only light perception or cannot see anything at all. People who have limited vision will be addressed in another section.
All input must be able to be performed on the keyboard. Output must be optionally spoken if a pair of headphones are inserted, or using another documented and easily found method. If a braille device is connected, output must be sent to that device. Braille support will depend on the Braille HID standard being adopted by more braille device vendors. There may be further work needed to bring braille to UEFI.