https://drafts.fxtf.org/compositing/#porterduffcompositingoperators_plus_darker

The definition here seems plain wrong. For instance:

co = max(0, 1 - αs x Cs + 1 - αb x Cb)
// Assuming
// αs = 1
// αb = 1
// Cs = 0.5
// Cb = 0
co = max(0, 1 - 1 x 0.5 + 1 - 1 x 0)
co = max(0, 1.5)
co = 1.5

max(0, x) is used, which suggests x will be a maximum of 1, but the definition produces values 0-2 (assuming pixel values are 0-1).

Safari supports the plus-darker value for mix-blend-mode.

The definition in Apple's docs matches the spec, so it's incorrect for the same reasons. The implementation is closed source, so I can't check how it actually works.

It seems like, on WebKit Linux (via cairo), plus-darker is an alias of darken, which doesn't match what I see in Safari. I might not be interpreting the source correctly.

We have not heard any use-cases for plus-darker, although maybe that will become clearer with correct documentation.