CSS border issue with THEAD TBODY and COLSPAN

Question

i have an issue with css/html table:
When I use thead and tbody tags with a colspan attribute, the bottom border of the header is divided.
The gap size is dependent of the th border width.

Did you have a solution to get a continuous border on header bottom (without removing thead and tbody) ?

JSFiddle example

table {
  border-collapse: collapse;
}

th {
  border: 4px solid red;
  border-bottom: 4px solid black
}

td {
  border: 4px solid blue;
}

thead tr {
  border-bottom: 5px solid green
}

table {
  border-collapse: collapse;
}

th {
  border: 4px solid red;
  border-bottom: 4px solid black
}

td {
  border: 4px solid blue;
}

thead tr {
  border-bottom: 5px solid green
}

with THEAD and TBODY but without COLSPAN
<table>
  <thead>
    <tr>
      <th>
        Column 1
      </th>
      <th>
        Column 2
      </th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>
        Content 1
      </td>
      <td>
        Content 2
      </td>
    </tr>
  </tbody>
</table>
<br /> COLSPAN with THEAD and TBODY <span style="background:yellow">(css bug in the middle of green border ?)</span>
<table>
  <thead>
    <tr>
      <th>
        Column 1
      </th>
      <th>
        Column 2
      </th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td colspan="2">
        Content 1 and 2 (merged cells)
      </td>
    </tr>
  </tbody>
</table>
<br /> COLSPAN without THEAD and TBODY
<table>
  <tr>
    <th>
      Column 1
    </th>
    <th>
      Column 2
    </th>
  </tr>
  <tr>
    <td colspan="2">
      Content 1 and 2 (merged cells)
    </td>
  </tr>
</table>

Answer 1

The corner rendering between collapsed borders does not seem well-specified, so it's not clear that this is actually a bug rather than just a variance in behaviour.

I did find a horrible workaround for Firefox, by creating a pseudo second row in the thead, and then hiding it, and also hiding the top border of the first tbody row like this:

thead:after {
    content:'';
    display:table-row;  /* see note below */
    position:absolute;
    visibility:hidden;
}

tbody tr:first-child td {
    border-top-width:0;
}

_{(Note that the display:table-row is just for show. In reality, the position:absolute causes the pseudo-element to be display:block regardless of whether the display property is set to table-row or left at its default inline. The table layout of the container will then cause that block to be wrapped in anonymous table objects of table-row and table-cell to form a well structured table.)}

table {
  border-collapse: collapse;
}

th {
  border: 4px solid red;
  border-bottom: 4px solid black
}

td {
  border: 4px solid blue;
}

thead tr {
  border-bottom: 5px solid green
}

table ~ table thead:after {
    content:'';
    position:absolute;
    visibility:hidden;
}
table ~ table tbody tr:first-child td {
    border-top-width:0;
}

with THEAD and TBODY but without COLSPAN
<table>
  <thead>
    <tr>
      <th>
        Column 1
      </th>
      <th>
        Column 2
      </th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>
        Content 1
      </td>
      <td>
        Content 2
      </td>
    </tr>
  </tbody>
</table>
<br /> COLSPAN with THEAD and TBODY <span style="background:yellow">(css bug in the middle of green border ?)</span>
<table>
  <thead>
    <tr>
      <th>
        Column 1
      </th>
      <th>
        Column 2
      </th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td colspan="2">
        Content 1 and 2 (merged cells)
      </td>
    </tr>
  </tbody>
</table>
<br /> COLSPAN without THEAD and TBODY
<table>
  <tr>
    <th>
      Column 1
    </th>
    <th>
      Column 2
    </th>
  </tr>
  <tr>
    <td colspan="2">
      Content 1 and 2 (merged cells)
    </td>
  </tr>
</table>