Report headers attribute referencing other <td> elements as unsupported

[WilcoFiers]

The headers attribute on td elements should not be allowed to reference other td elements by the td-headers-attr rule.

The following should fail this rule:

<table>
  <tr>
    <td id="hdr1">Hello</td>
    <td id="hdr2">World</td>
  </tr>
  <tr>
    <td headers="hdr1">A</td>
    <td headers="hdr2">1</td>
  </tr> 
  <tr>
    <td headers="hdr1">B</td>
    <td headers="hdr2">Wor2ld</td>
  </tr>
</table>

While HTML doesn't explicitly prohibit this, this does not seem to work with any screen reader running against a Chromium browser.

[WilcoFiers]

The way to solve this is by adding code to test the node name in lib/checks/tables/td-headers-attr-evaluate.js. We'll need both unit tests for the check (test/checks) and integration tests (test/integration/rules/)

[jlin95]

Hi @WilcoFiers , I'd like to take this if it's not assigned yet.