ast line box in the normal flow,
  > unless it has either no in-flow line boxes or if its overflow property has a computed
  > value other than visible, in which case the baseline is the bottom margin edge.

  Here we have 2 inline-blocks. The 1st one is completely empty, so it baseline is its
  bottom margin edge. The 2nd one has an empty span followed by a block.

  The span would typically be placed in a line box. However, it's a phantom line box.
  From https://drafts.csswg.org/css2/#inline-formatting
 