Using PROTO_CANONICAL will use the full https:// prefix instead of the "protocol-relative" // prefix for wikilinks. This costs bytes (!) but works around Safari bugs with "visited" status for protocol-relative links (T425211: Link colours don't change to purple in Safari on Parsoid-served pageviews).
Only Parsoid output goes through the ExpandRelativeAttrs post-processing stage. Some discussion of alternatives is in T350952#12091784.