Check 5 (spec 3.10): a :history state's placement, default transition, and type. Four independent facts, per :history state in the document:

• {:history_bad_parent, id, parent_kind} when the parent is not a compound <state> or a <parallel> (Context.compound?/1, Decision
  6. • the document root, a :final, and another :history are all illegal parents.
• Statifier.Validator.Checks.DefaultTransition's shared sub-check with owner: {:history, id} - required, exactly one, a non-null target, no event, no cond (spec 3.10 requires the default transition unconditionally, stricter than a looser "when present" reading).
• {:initial_not_descendant, target, parent_id} when a resolved default target is not a descendant of the history's own parent (spec 3.10)
  • not of the history state itself, which has no descendants of its own to test against. Reuses check 3's constructor: this is the same shape of mistake, just against a different parent. Skipped when the target does not resolve at all (Statifier.Validator.Checks.Targets already reported that) or when the parent is not compound - a non-compound parent has already been reported by :history_bad_parent, and testing descendancy against it (an id that may not even exist, for the document root) would be a second, meaningless error for the same mistake.
• {:history_bad_type, raw} when type was written and, sliced back out of context.source, is neither "shallow" nor "deep". Lowering silently maps any out-of-range value to the :shallow default (Residual Note 2), so the atom on %State{} alone cannot tell type="shallow" from type="sideways" - only the raw source text can.