↳ In reply to

issue 3237 of GitHub project “csswg-drafts”

The CSS Scrollbars editor’s draft has an update to the Computed value for scrollbar-color of:
specified keyword or two computed colors

which reflects the above resolution as of 2018-10-23: https://github.com/w3c/csswg-drafts/issues/3237#issuecomment-432224775
That just leaves adding it to the changes section.

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