Guidelines
The rules behind the defaults.
One primary action per view
Competing primary buttons slow the decision. Keep one, make the rest quieter.
Defaults do the work
Most people keep what they are given, so a default must be the safe, common choice.
Show less, reveal on demand
Start with what most people need. Put the rest one step away, not on another page.
Say what happened and what to do
An error names the problem, the reason and the fix, next to the field it concerns.
Respond within 100 ms
Under that, a change feels instant. For longer waits show progress, not a blank.
Targets you can hit
Controls grow to 44 px on touch screens, with room between them.
Colour is never alone
Status always has an icon, shape or word as well, so it survives colour blindness and bad screens.
No tricks
No preselected upsells, no hidden cancel. Trust is worth more than one conversion.
Citations pending
Sources are being checked and will be linked here.