I think emulators are allowed, but can you use graphical enhancements such as upscaled resolution, per-pixel lighting, anti aliasing, texture filtering, and other little details that make the game look better?

Ask in the forum for whatever game it is that you’re running. There are no site-wide rules about this.

Generally when using emulators for speedrunning you should use settings that make the game behave as accurately as possible (meaning that, ideally, it would be identical to playing the game on a real console). But yeah some games are fairly lax on emulator rules so as always check the game rules for whatever game(s) you're planning on running to see what (if any) emulators/emulator settings are allowed.