The kind of art which can be considered avant-garde depends upon its context-- its time and place.
Both dada and surrealism, for instance, were responses to the chaos of World War I. They were of their time. The two arts movements were intentionally and specifically revolutionary, seeking a cultural revolution and also political revolution. They're understandable as avant-garde art within their historical context.
An avant-garde attacks the cultural precepts, institutions, and lauded icons of its time. It stands out against the mundane crowd. Otherwise its rebellion has no meaning-- isn't rebellion.