Minimum standard. Without achieving "A" standard, it is assumed that there could be a group of people who might not be able to browse the website comfortably.
The desirable standard. If "AA" standard is achieved, there will be less cases of difficulty in browsing websites.
Advanced criteria. A website would be the most accessible to browse if "AAA" standard is achieved.
Levels of conformanceA
KEYENCE CORPORATION OF AMERICA
Spotify for Developers
Starbucks Coffee Company
Musée du Louvre