The danger of limiting the ability to style scrollbars is web devs will again resort to dumping native scrollbars for custom re-implementations which usually worsens the user experience more than when they're styled using CSS. This applies more to pages using very specific, far from native design. See #1969, someone in a call mentioned a page for an 8-bit game trying to ensure consistent styling etc. Width & color will not be enough to cover them all.
I hope this is taken into account during the decision making.
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