It seems to me the current spec requires PaintClass to have `overflow()` method, otherwise `registerPaint` would fail. However, with this requirement, it seems the Example 1 in the spec is invalid. The example there doesn't have the `overflow()` method. That makes me wonder, do we really want to force authors to add this method, even if they don't want to change anything? (BTW, it seems in PaintClass'd declaration, `overflow()` have `void` as its return type, which doesn't seem to make sense.)