Scrollbar isn't like other default apps in elementary OS.
The scrollbar looks a bit different in Epiphany than it does for every elementary OS default app. It also behaves differently in that it doesn't maximize on hover like all default eOS apps.
Epiphany version 3.28.1 (Unsure how to check webkit version in elementary OS as there is no about dialog available) elementary OS 5.0 Juno(64-bit), built on Ubuntu 18.04, Linux 4.15, GTK+3.22.30