[csswg-drafts] [css-tables-3] How table borders should be painted at the corner? (border-collapse: collapse) (#13406)

karlcow has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-tables-3] How table borders should be painted at the corner? (border-collapse: collapse) ==
Currently the painting at the corner of collapse borders differs in between Chrome/Safari and Firefox.

This is easily visible with larger borders. 
<img width="3160" height="759" alt="Image" src="https://github.com/user-attachments/assets/0913ca32-29e5-4660-ac2f-e63bf713f8c9" />

As you can see, chrome and Safari adds the opacity on top of each other, while Firefox doesn't have this rendering because the corners are painted in diagonal (like a real wood painting frame assemblage)

But which is correct?
I didn't find anything in the specification explaining how it should be done. 
https://drafts.csswg.org/css-tables-3/#conflict-resolution-for-collapsed-borders



https://codepen.io/webcompat/pen/XJKVezR

```
<style>
  td {
    width: 50px;
    height: 50px
  }

  table {
    border-collapse: collapse;
    margin: 10px
  }
</style>
<table>
  <tbody>
    <tr>
      <td style="
           border-left: 20px solid rgba(0, 127, 0, 0.5); 
           border-bottom: 20px solid rgba(0, 127, 0, 0.5);"></td>
    </tr>
  </tbody>
</table>

<table>
  <tbody>
    <tr>
      <td style="
           border-left: 20px solid rgba(0, 127, 0, 0.5); 
           border-bottom: 20px solid rgba(127, 0, 0, 0.5);"></td>
    </tr>
  </tbody>
</table>
```

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13406 using your GitHub account


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Received on Wednesday, 28 January 2026 14:31:46 UTC