prev and next <scroll-button-direction> values make ::scroll-button() selectors layout dependent, but the spec doesn't define the time and the way browsers should match it.

I want to discuss if we really need them at all, and then either remove them or better specify what and when to do.
This layout dependency for CSS selectors is a new thing, so, I want to also discuss the complexity vs the usefullness of it.

cc: @tabatkins @flackr @lilles