How CSS scroll-state queries can help users understand that a box is scrollable. The power of scroll-state queries here is the ability to dynamically apply an affordance.
A scrollable area with no visual affordances unless scrollbars are set to visible in the OS.
Border, background and cutoff visual affordances. No scroll-state queries as the effect is permanent.
A shadow visual affordance to indicate scroll
An arrow visual affordance to indicate scroll
A scroll-to-top affordance