[fonts-4] Make it hopefully clearer how 'base-palette' defaults.

Author: tabatkins

css-fonts-4/Overview.bs

@@ -2828,9 +2828,8 @@ in the color palette represented by this ''@font-palette-values'' block.
 </div>
 
 If this descriptor is not present in the ''@font-palette-values'',
-a value of ''0'' is used.
-If the font does not contain a palette at the index of the value of 'base-palette',
-a value of ''0'' is used.
+or if the font does not contain a palette at the index of the value of 'base-palette',
+it behaves as if ''0'' were specified.
 If a font does not contain any color palettes,
 no colors are included in the initial color palette
 represented by this ''@font-palette-values'' rule.