rubbernecker vs viewer

rubbernecker and viewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
rubbernecker	Yes	No	No	No
viewer	Yes	No	No	No

As nouns, viewer is a hypernym of rubbernecker; that is, viewer is a word with a broader meaning than rubbernecker:

rubbernecker: a person who stares inquisitively
viewer: a close observer; someone who looks at something (such as an exhibition of some kind)

Other hypernyms of rubbernecker include looker, spectator, watcher, witness.

rubbernecker (noun)	viewer (noun)
a person who stares inquisitively	an optical device for viewing photographic transparencies
	a close observer; someone who looks at something (such as an exhibition of some kind)