“So what’s is going on? Well, CSS colors are understood to be in the sRGB color space, but the formula used is intended for linear RGB”
GitHub picks black or white label text with a luminance formula meant for linear RGB and feeds it sRGB values instead. Near the 0.453 threshold the CSS math stops snapping and renders grey text, like #929292 on teal. The skew pushes too many labels toward black text. One of the biggest developer platforms in the world skipped a gamma conversion that fits in a few lines of Python.