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.
The White House
Levels of conformanceAA
Tokio Marine & Nichido Fire Insurance
SAKATA SEED CORPORATION
Levels of conformanceA
BBC British Broadcasting Corporation