Why does overflow:hidden not work in a <td>?

Here is the same problem.

You need to set table-layout:fixed and a suitable width on the table element, as well as overflow:hidden and white-space: nowrap on the table cells.

Examples

Fixed width columns

The width of the table has to be the same (or smaller) than the fixed width cell(s).

With one fixed width column:

* {
  box-sizing: border-box;
}
table {
  table-layout: fixed;
  border-collapse: collapse;
  width: 100%;
  max-width: 100px;
}
td {
  background: #F00;
  padding: 20px;
  overflow: hidden;
  white-space: nowrap;
  width: 100px;
  border: solid 1px #000;
}

<table>
  <tbody>
    <tr>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
    </tr>
    <tr>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
    </tr>
  </tbody>
</table>

With multiple fixed width columns:

* {
  box-sizing: border-box;
}
table {
  table-layout: fixed;
  border-collapse: collapse;
  width: 100%;
  max-width: 200px;
}
td {
  background: #F00;
  padding: 20px;
  overflow: hidden;
  white-space: nowrap;
  width: 100px;
  border: solid 1px #000;
}

<table>
  <tbody>
    <tr>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
    </tr>
    <tr>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
    </tr>
  </tbody>
</table>

Fixed and fluid width columns

A width for the table must be set, but any extra width is simply taken by the fluid cell(s).

With multiple columns, fixed width and fluid width:

* {
  box-sizing: border-box;
}
table {
  table-layout: fixed;
  border-collapse: collapse;
  width: 100%;
}
td {
  background: #F00;
  padding: 20px;
  border: solid 1px #000;
}
tr td:first-child {
  overflow: hidden;
  white-space: nowrap;
  width: 100px;
}

<table>
  <tbody>
    <tr>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
    </tr>
    <tr>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
      <td>
        This_is_a_terrible_example_of_thinking_outside_the_box.
      </td>
    </tr>
  </tbody>
</table>

That's just the way TD's are. I believe It may be because the TD element's 'display' property is inherently set to 'table-cell' rather than 'block'.

In your case, the alternative may be to wrap the contents of the TD in a DIV and apply width and overflow to the DIV.

<td style="border: solid green 1px; width:200px;">
    <div style="width:200px; overflow:hidden;">
        This_is_a_terrible_example_of_thinking_outside_the_box.
    </div>
</td>

There may be some padding or cellpadding issues to deal with, and you're better off removing the inline styles and using external css instead, but this should be a start.

Apply CSS table-layout:fixed; (and sometimes width:<any px or %>) to the TABLE and white-space: nowrap; overflow: hidden; style on TD. Then set CSS widths on the correct cell or column elements.

Significantly, fixed-layout table column widths are determined by the cell widths in the first row of the table. If there are TH elements in the first row, and widths are applied to TD (and not TH), then the width only applies to the contents of the TD (white-space and overflow may be ignored); the table columns will distribute evenly regardless of the set TD width (because there are no widths specified [on TH in the first row]) and the columns will have [calculated] equal widths; the table will not recalculate the column width based on TD width in subsequent rows. Set the width on the first cell elements the table will encounter.

Alternatively, and the safest way to set column widths is to use <COLGROUP> and <COL> tags in the table with the CSS width set on each fixed width COL. Cell width related CSS plays nicer when the table knows the column widths in advance.