ee lines below. The middle line should have a
  yellow background (but otherwise be exactly like the two others,
  i.e. no extra height or anything like that)</p>
A B C D E F G
<div style="writing-mode:vertical-rl; background:yellow;">
  <div style="writing-mode:horizontal-tb;">
    A B C D E F G
  </div>
</div>
A B C D E F G
