Yosei, the Morning Star

• WWizards of the Coast2017-11-17
  Official Ruling
  If the player becomes an illegal target, the target permanents are tapped, but the player doesn't skip their untap step.
• WWizards of the Coast2017-11-17
  Official Ruling
  If more than one effect instructs a player to skip their next untap step, such as if Yosei dies more than once in a turn, that player skips that many untap steps. This is different from effects that state that something doesn't untap during an untap step.
• WWizards of the Coast2017-11-17
  Official Ruling
  If a permanent won't untap during its controller's next untap step, that effect waits to apply until its controller has an untap step that isn't skipped.
• WWizards of the Coast2017-11-17
  Official Ruling
  If any of the targets of Yosei's ability are illegal, they won't be tapped.
• WWizards of the Coast2017-11-17
  Official Ruling
  If all of the permanents targeted by Yosei's ability become illegal targets, nothing becomes tapped, but the target player still skips their untap step.

Rulings via Scryfall