[css-borders][css-tables] The rowspan cell bottom-border takes the color of the last border shared adjacent cell

[karlcow]

I just fixed a difference of behavior in between WebKit and Blink/Gecko.
I will be exporting to WPT the testcase.

Before on Safari

On Chrome/Firefox. And soon on Safari

The 3 rendering engines will behave the same.

The markup.

<style>
  table {
    border-collapse: collapse;
  }

  td {
    width: 100px;
    height: 50px;
    border: 1px solid black;
  }

  /* Different border colors for each row */
  #test tr:nth-child(1) {
    border-bottom: 5px solid blue;
  }

  #test tr:nth-child(2) {
    border-bottom: 5px solid green;
  }

  #test tr:nth-child(3) {
    border-bottom: 5px solid red;
  }
</style>
<p>The cell with rowspan=3 should have a red bottom border (from row 3), not blue (from row 1).</p>
<table id="test">
  <tr>
    <td rowspan="3">Rowspan 3</td>
    <td>Cell B</td>
  </tr>
  <tr>
    <td>Cell C</td>
  </tr>
  <tr>
    <td>Cell D</td>
  </tr>
</table>

[Loirooriol]

Servo:

[karlcow]

WPT export web-platform-tests/wpt#57442