@type t() :: %Expresso.Presenter.Projection.Mark{
  __spark_metadata__: term(),
  attribute: String.t(),
  key: :slide | :index,
  scroll: boolean(),
  selector: String.t(),
  values: [{String.t(), atom(), integer()}]
}

An attribute on the elements whose number agrees with a field of the state

key names the number of an element: :slide reads data-slide, and :index reads data-index. Each value holds the text of the attribute, a field of the state and an offset.