Epiphany handles Shift+Tab even when the page should be having control of it, whereas Tab works
This happens with the egui demo which uses WASM to show the whole UI. Egui provides keyboard navigation with Tab and Shift+Tab, which works fine in Firefox. Currently it seems the expected behaviour that egui provides is to capture all Tab and Shift+Tab key combinations for its own use, without having the keyboard focus come back to the browser's header bar (but that's most likely something that egui should handle, not a problem in the browsers). But while tabbing with the Tab key in epiphany works as expected, the Shift+Tab key is never used by the egui page but directly used to tab through epiphany's header bar buttons.
What makes me think it's a bug and not a feature is that for regular Tab it works as expected, whereas Shift+Tab doesn't. I don't think that's a webkit content bug though, as epiphany has some code like https://gitlab.gnome.org/GNOME/epiphany/-/blob/master/src/ephy-window.c#L617 that might be breaking it here.