Thescrollbar-colorproperty controls the two colors of a scrollbar: thethumb colorand thetrack color.scrollbar-coloris part of theCSS Working Group’s Scrollbars Module Level 1 draft, which is still a work in progress as of this writing. ...
Because we put the responsibility of ensuring good contrast on the author, I think leaving it up to the UA is not appropirate. This is related to, but possibly different from the similar question about accent colors discussed in#9852: the difference arises because in the accent-color's case...