<!DOCTYPE HTML>
<html>
<head>
<title>Percent widths on table cells</title>
<style type="text/css">

table {
  border-spacing: 3px 5px;
  padding: 2px 3px 4px 5px;
}

td {
  border: none;
  padding: 0;
}

</style>
</head>
<body>

<table><tr>
<td style="background: yellow">xxxxx</td>
<td style="background:aqua" width="100%">x</td>
</tr></table>

<table><tr>
<td style="background:aqua" width="100%">x</td>
<td style="background: yellow">xxxxx</td>
</tr></table>

</body>
</html>