Black and white provide the endpoints of opaque sRGB relative luminance. That makes the pair a useful test fixture for checking an implementation, although a maximum numerical ratio alone does not decide typography, hierarchy or whether a design is comfortable in every context.
Normalize both endpoint colors
Enter #000 or #000000 as foreground and #FFF or #FFFFFF as background. Shorthand expands to six digits, producing channel triplets 0,0,0 and 255,255,255. Swapping the fields preserves the ratio because the formula always assigns the greater luminance to L1. The roles still remain visible so the preview and copied result describe the intended foreground and background.
Luminance zero and one produce 21
Every black channel linearizes to zero, so black luminance is 0. Every white channel linearizes to one; the three weights sum to 1, so white luminance is 1. Substitute them into (L1 + 0.05) / (L2 + 0.05): 1.05 divided by 0.05 equals exactly 21. The visible result is therefore 21.00:1 with no boundary ambiguity.
Every displayed numerical reference passes
A raw ratio of 21 exceeds AA normal 4.5, AA large 3, AAA normal 7, AAA large 4.5 and the non-text 3:1 reference. That confirms the arithmetic for this pair only. It does not prove that every page color, state, control, image or requirement has been reviewed. Use the example as a known endpoint and retain the tool’s narrow conclusion.