Tested with iPhone, VoiceOver on Safari:
- error message can't have HTML attribute hidden - otherwise error was not announced when we focus on the field
- error message can't have display:none - otherwise error was not announced when we focus on the field
- error message can have opacity:0.01; - error will be announced when we focus on the field
- error message can have visibility:hidden; - error will be announced when we focus on the field