The inner voices

In a four-part texture, the outer voices get the attention. The soprano carries the tune and the bass carries the harmony, and if you play only those two the piece is still recognisable. The alto and tenor are the ones you cannot hum afterwards.

They are also the ones that make it a piece rather than a tune with chords under it. Take them out and nothing is wrong, exactly. It is just thin.

Interfaces have inner voices, and they are the ones nobody looks at: focus order, live region announcements, the accessible name on the icon button, the order the DOM is actually in as opposed to the order CSS grid makes it look. The ones you cannot see in a screenshot.

They fail silently, which is the problem

An outer-voice bug is loud. The button is the wrong colour; someone files it within a day. An inner-voice bug is silent by construction: focus jumps to the top of the document after a delete, and the only people who find out are the people navigating by keyboard, who have learned not to bother reporting it.

So the pairs I said to enumerate two variations ago need a second column. Not just what shares the bottom of the screen, but what shares focus, what speaks, what order the document is really in.

A test you can actually run

Unplug the mouse. Do the main task. Then turn the screen off and do it again with a screen reader.

That second one is uncomfortable and slow and you will not enjoy it, and it will find things no audit tool finds, because the tool checks single voices and you are finally listening to the texture.