3 x 3より大きいテーブル内の空でないすべての<td>要素には、関連付けられたテーブルヘッダーが必要です。

<table class="data">
<caption>
Greensprings Running Club Personal Bests
</caption>
  <thead>
    <tr>
     <th scope="col">Name</th>
      <th scope="col">1 mile</th>
      <th scope="col">5 km</th>
      <th scope="col">10 km</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th scope="row">Mary</th>
      <td>8:32</td>
      <td>28:04</td>
      <td>1:01:16</td>
    </tr>
    <tr>
      <th scope="row">Betsy</th>
      <td>7:43</td>
      <td>26:47</td>
      <td>55:38</td>
    </tr>
    <tr>
      <th scope="row">Matt</th>
      <td>7:55</td>
      <td>27:29</td>
      <td>57:04</td>
    </tr>
    <tr>
      <th scope="row">Todd</th>
      <td>7:01</td>
      <td>24:21</td>
      <td>50:35</td>
    </tr>
  </tbody>
</table>

<table class="data complex" border="1">
<caption>
Example 2 (column group headers): 
</caption>
<tr>
<td rowspan="2"><span class="offscreen">empty</span></td>
<th colspan="2" id="females2">Females</th>
<th colspan="2" id="males2">Males</th>
</tr>
<tr>
<th width="40" id="mary2">Mary</th>
<th width="35" id="betsy2">Betsy</th>
<th width="42" id="matt2">Matt</th>
<th width="42" id="todd2">Todd</th>
</tr>
<tr>
<th width="39" id="mile1_2">1 mile</th>
<td headers="females2 mary2 mile1_2">8:32</td>
<td headers="females2 betsy2 mile1_2">7:43</td>
<td headers="males2 matt2 mile1_2">7:55</td>
<td headers="males2 todd2 mile1_2">7:01</td>
</tr>
<tr>
<th id="km5_2">5 km</th>
<td headers="females2 mary2 km5_2">28:04</td>
<td headers="females2 betsy2 km5_2">26:47</td>
<td headers="males2 matt2 km5_2">27:29</td>
<td headers="males2 todd2 km5_2">24:21</td>
</tr>
<tr>
<th id="km10_2">10 km</th>
<td headers="females2 mary2 km10_2">1:01:16</td>
<td headers="females2 betsy2 km10_2">55:38</td>
<td headers="males2 matt2 km10_2">57:04</td>
<td headers="males2 todd2 km10_2">50:35</td>
</tr>gt;