I want the link to fill the entire containing cell, including vertically.

I have basically this:

<table>
<tr>
<td style="padding:0px; border:1px solid green;"><a href="https://google.com" style="display:block; margin:0px; height:100%; border:1px solid red;">Text</a></td>
<td>Multi-Line<br />Text</td>
</tr>
</table>

If I put an explicit height ( height:100px; )on the containing cell then the internal element fills that height. Otherwise I get a gap.

Fiddle: https://jsfiddle.net/3c91Lkpz/

Edit: None of the suggestions quite worked right, so I convinced them to accept a different tack.

Edit 2: Following up for future people. I did find the following.

<style type="text/css">
td.biglink {
  position: relative;
  background-color: #ccc;
}

td.biglink:hover {
  background-color: #eee;
}

td.biglink a::after {
  content: '';
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  top: 0;
  z-index: 0;
}
</style>
<table>
 <tr>
  <td class="biglink"><a href="http://www.google.com/">Cell 1<br>second line</a></td>
  <td>Cell 2 no link<br><br><br><br></td>
  <td class="biglink"><a href="http://www.google.com/">Cell 3</a></td>
 </tr>
</table>

The background color change is the cell background, otherwise the ::after background covers the text. You can do a semi-transparent hover if you don't want the cell to do it. The benefit is reducing the need of a class on the cell. The problem is that it still overlays the text.

<style type="text/css">
td {
  position: relative;
}

td a::after {
  content: '';
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  top: 0;
  z-index: 0;
}
td a:hover::after {
  background: rgba(170,234,204,0.5);
}
</style>
<table>
 <tr>
  <td><a href="http://www.google.com/">Cell 1<br>second line</a></td>
  <td>Cell 2 No Link<br><br><br><br></td>
  <td><a href="http://www.google.com/">Cell 3</a></td>
 </tr>
</table>