lso have some border/padding that UAs will need to account for
     * when computing the stretched children's content-box sizes; but these
     * don't reduce our `data-expected-{width,height}` expectations, since
     * those correspond to the border-box size. */
    border: 3px solid blue;
    padding: 2px;

    block-size: stretch;
    inline-size: 20px;
    background: fuchsia;
  }
  .htb { writing-mode: horizontal-tb; }
  .vlr { writing-mode: vertical-lr;   }
  .vrl { writing-mode: vertical-rl;   }
</s