An interface is a conversation, and this section is about holding up your end of it. Every press, hover, tap, keystroke and mistake is someone asking the interface something. The answer has to come quickly, it has to be clear, and it has to work however they got there, whether that's a mouse, a finger, a keyboard or a screen reader.
Most of this goes unnoticed when it's done well. A button that gives way under your finger. A submenu that stays open while you reach for it. A form that tells you what's wrong instead of greying out the button and leaving you to guess. People only notice these things when they're missing.
How a button should feel the moment you press it.
Hover tells you something is interactive. It should never be the only way to find out.
Why some submenus close as you reach for them, and the invisible shape that fixes it.
What you can tap and what you can see don't have to be the same size.
Focus rings aren't ugly. Badly designed focus rings are.
Power users and screen reader users both live on the keyboard. Design for it, not around it.
Formatting as people type is helpful right up until the caret jumps.
A greyed-out button tells people they're wrong without telling them why.
"Are you sure?" trains people to click yes. Undo lets them actually change their mind.