Two Rows in thead, Can't Set Column Widths in Second

I have a table with two rows of headings. I want to set the widths of some of the columns in the second row. I set up a Codepen to test it out. I find that if I remove the first row of headings, it takes the column widths I've specified without a problem. But with the first row of headings, it ignores the widths of the columns in the second row. :(

Please note that I have div's within the th's so I can style the borders.

Here's the Code:

body {
  padding: 10px;
}

.data-table {
  width: 100%;
  border-spacing: 0 4px;
  border-collapse: separate;
  table-layout: fixed;
}/* Remove For SCSS*/
  .title-11, .title-21 {
    width: 40px;
  }
  .title-22 {
    width: 100px;
  }
  .title-24 {
    width: 100px;
  }

  thead tr th {
    border-collapse: separate; 
    border-spacing: 0 5px;
  }
   .title {
     /*
     background: linear-gradient(to right, transparent 0%, #bbb 0%,
       #bbb calc(100% - 10px), 
       transparent calc(100% - 10px)) 0 99% / 100% 1px 
       no-repeat; 
     */
     text-align: left; 
     padding-right: 10px;
  }

  .div-title {
    border-bottom: 1px solid #bbb;
  }
  .hdr-field {
    width: 150px;
  }
  tr .title:last-child {
    padding-right: 0;
  }
  .side-title {
    transform: rotate(-90deg);
    width: 25px;
  }
  .fieldname {
    width: 130px;
  }
  .fieldvalue.dest-data input[type=text] {
    width: 100%;
  }
  .bodySection {
    border-bottom: 10px solid #bbb;
    margin-bottom: 10px;
  }
  tr {
    // border-bottom: 10px solid #bbb;
  }
/*}*//* For SCSS*/

<table class="data-table">
  <thead>
    <tr>
      <th class="title-11"></th>
      <th colSpan="2" class="title title-12">
        <div class="div-title">Source</div>
      </th>
      <th colSpan="2" class="title title-13">
        <div class="div-title">Destination</div>
      </th>
    </tr>
    <tr>
      <th class="title-21"></th>
      <th colSpan="1" class="fieldname title title-22">
        <div class="div-title hdr-field">Field</div>
      </th>
      <th colSpan="1" class="title title-23">
        <div class="div-title">Value</div>
      </th>
      <th colSpan="1" class="fieldname title title-24">
        <div class="div-title hdr-field">Field</div>
      </th>
      <th colSpan="1" class="title title-25">
        <div class="div-title">Value</div>
      </th>
    </tr>
  </thead>
  <tbody class="bodySection">
    <tr>
      <td rowSpan="2" class="side-title">Name</td>
      <td class="fieldname src-data">Short Name</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Short Name</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store" /></td>
    </tr>
    <tr>
      <td class="fieldname src-data">Long Name</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Long Name</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store" /></td>
    </tr>
  </tbody>
  <tbody class="bodySection">
    <tr>
      <td rowSpan="2" class="side-title">Name2</td>
      <td class="fieldname src-data">Short Name2</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Short Name2</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store2" /></td>
    </tr>
    <tr>
      <td class="fieldname src-data">Long Name2</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Long Name2</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store2" /></td>
    </tr>
  </tbody>
</table>

So just to clarify: the question is how to make the Field columns fixed width, while leaving the Value columns to find their own level.

Thank you.

Solution

I can't quite understand why and how table lay-outs are calculated, because the algorythm seems pretty complex.

However, you can achieve the desired lay-out by using a good ol' <colgroup>. There's some not-too-hard to digest info about how to use them in the HTML4 spec | Calculating column width. Note that this also holds true for the new spec (col and colgroup are not deprecated).

Stick this in between <table> and <thead> in your fiddle:

<colgroup>
  <col>
  <col width="150">
  <col>
  <col width="150">
  <col>
</colgroup>

DEMO working:

body {
  padding: 10px;
}

.data-table {
  width: 100%;
  border-spacing: 0 4px;
  border-collapse: separate;
  table-layout: fixed;
}/* Remove For SCSS*/
  .title-11, .title-21 {
    width: 40px; /* Wont be applied */
  }
  .title-22 {
    width: 100px; /* Wont be applied */
  }
  .title-24 {
    width: 100px; /* Wont be applied */
  }

  thead tr th {
    border-collapse: separate; 
    border-spacing: 0 5px;
  }
   .title {
     /*
     background: linear-gradient(to right, transparent 0%, #bbb 0%,
       #bbb calc(100% - 10px), 
       transparent calc(100% - 10px)) 0 99% / 100% 1px 
       no-repeat; 
     */
     text-align: left; 
     padding-right: 10px;
  }

  .div-title {
    border-bottom: 1px solid #bbb;
  }
  .hdr-field {
    width: 150px;
  }
  tr .title:last-child {
    padding-right: 0;
  }
  .side-title {
    transform: rotate(-90deg);
    width: 25px;
  }
  .fieldname {
    width: 130px;
  }
  .fieldvalue.dest-data input[type=text] {
    width: 100%;
  }
  .bodySection {
    border-bottom: 10px solid #bbb;
    margin-bottom: 10px;
  }
  tr {
    // border-bottom: 10px solid #bbb;
  }
/*}*//* For SCSS*/

<table class="data-table">
  <colgroup>
    <col width="40">
    <col width="100">
    <col>
    <col width="100">
    <col>
  </colgroup>
  <thead>
    <tr>
      <th class="title-11"></th>
      <th colSpan="2" class="title title-12">
        <div class="div-title">Source</div>
      </th>
      <th colSpan="2" class="title title-13">
        <div class="div-title">Destination</div>
      </th>
    </tr>
    <tr>
      <th class="title-21"></th>
      <th class="fieldname title title-22">
        <div class="div-title hdr-field">Field</div>
      </th>
      <th class="title title-23">
        <div class="div-title">Value</div>
      </th>
      <th class="fieldname title title-24">
        <div class="div-title hdr-field">Field</div>
      </th>
      <th class="title title-25">
        <div class="div-title">Value</div>
      </th>
    </tr>
  </thead>
  <tbody class="bodySection">
    <tr>
      <td rowSpan="2" class="side-title">Name</td>
      <td class="fieldname src-data">Short Name</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Short Name</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store" /></td>
    </tr>
    <tr>
      <td class="fieldname src-data">Long Name</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Long Name</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store" /></td>
    </tr>
  </tbody>
  <tbody class="bodySection">
    <tr>
      <td rowSpan="2" class="side-title">Name2</td>
      <td class="fieldname src-data">Short Name2</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Short Name2</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store2" /></td>
    </tr>
    <tr>
      <td class="fieldname src-data">Long Name2</td>
      <td class="fieldvalue src-data"My Store/td>
      <td class="fieldname dest-data">Long Name2</td>
      <td class="fieldvalue dest-data"><input type="text" value="My Store2" /></td>
    </tr>
  </tbody>
</table>