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How were the WCAG contrast ratios determined

for

From: Sean Curtis
Date: Feb 11, 2016 7:30PM

Does anyone have any historical information about how the ratios for A, AA,
and AAA were determined? Were they based on the contrast of screens, or on
how light and vision works? I couldn't find any information about how these
were initially determined. Can anyone fill me in?

Cheers,

Sean