would still be possibly legal on the basis that if it's not illegal then it's legal - in British jurisprudence tradition at least https://en.wikipedia.org/wiki/Everything_which_is_not_forbid... - namely it's not law that impedes it (also in some places there's posthumous marriage)
There are also people who are considered dead by the bureaucratic system, but physically alive. Usually because of clerical errors that are sometimes surprisingly hard to resolve. In this context the wife of the man would be considered a widow in many contexts, despite her man being alive.
