[css-scrollbars] Fix computed value of scrollbar properties

Fix #3149.

Author: upsuper

css-scrollbars-1/Overview.bs

@@ -70,7 +70,7 @@ Name: scrollbar-color
 Value: auto | dark | light | <<color>>{2}
 Initial: auto
 Inherited: yes
-Computed value: two computed colors or the keyword auto
+Computed value: keyword or two computed colors
 Animation type: <a href="https://www.w3.org/TR/css3-transitions/#animtype-color">color</a>
 Applies to: boxes to which 'overflow' applies
 Percentages: n/a
@@ -158,7 +158,7 @@ Name: scrollbar-width
 Value: auto | thin | none | <<length>>
 Initial: auto
 Inherited: no
-Computed value: absolute length or the keyword auto.
+Computed value: keyword or absolute length
 Animation type: <a href="https://drafts.csswg.org/css3-transitions/#animtype-length">length</a>
 Applies to: boxes to which 'overflow' applies
 Percentages: n/a