Why the contrast ratio is 2.5:1?

Why the contrast ratio is 2.5:1?
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#1

hi, I am learning the part named “applied accessibility”, and I feel confused about “contrast ratio”.

body {
color: hsl(0, 55%, 20%);
background-color: hsl(120, 25%, 35%);
}

Current combination of a greenish background-color with maroon text color has a 2.5:1 contrast ratio.

  • Why the contrast ratio is 2.5:1?