[css-position-3][css-align-3] justify-self on abspos children of flexbox

[fantasai]

Currently the Flexbox spec defines that justify-content on the flex container affects the static position of an abspos flex contaner child. But justify-self on the child has no effect. We're updating the abspos rules to apply alignment across most layout modes; probably we should do that for Flexbox, too, but

• A) How would that interact with the justify-content rule currently in place? (Maybe only justify-self: normal follows parent's justify-content?)
• B) Is it Web-compatible to do this?

Raised by @cbiesinger on 2020-04-29