ck5 caveats documentation: The insertTable plugin should be disabled in your text format unless you're fine with strict tables and do not use th elements in your tbody

Problem/Motivation

We switched from CK4 to CK5 as part of our Drupal 10 to Drupal 11 upgrade and we hit a nasty surprise on one of our more complex sites that has elements in a <tbody> such as <tr><th>

🐛 [upstream] [Table] CKEditor 5 editing table with rowspans removes header property Postponed: needs info
🐛 [upstream] [Table] CKEditor 5 changes and keeps only a single , which impedes screen reader accessibility of HTML tables Postponed

Upstream issues:

• https://github.com/ckeditor/ckeditor5/issues/14911
• https://github.com/ckeditor/ckeditor5/issues/17962
• https://github.com/ckeditor/ckeditor5/pull/17870 Didn't prevent the markup mangling situation we found relating to table th elements being moved
• https://github.com/ckeditor/ckeditor5/issues/10916 Ideally we would have this option available but for some reason the ck5 folks won't provide the ability to disable their filtering!

Steps to reproduce

Switch from ck4 to ck5

use this test html for example, see the before and after:

<table>
<thead>
<tr>
<td class="text-right"><b>Yearly product launches</b></td>
<td><b>6</b></td>
<td><b>8</b></td>
<td><b>20</b></td>
<td><b>14</b></td>
</tr>
</thead>
<tbody>
<tr><th colspan="6" id="tcl325">Top five brands</th></tr>
<tr>
<td class="text-left">UMMEAT</td>
<td>20</td><td>30</td><td>32</td><td>8</td>
</tr>
<tr>
<td class="text-left">Mamp;5 Food Plant Kitchen</td>
<td>20</td><td>30</td><td>32</td><td>8</td>
</tr>
<tr>
<td class="text-left">Alpro Barista</td>
<td>20</td><td>30</td><td>32</td><td>8</td>
</tr>
<tr>
<td class="text-left">Alpro</td>
<td>20</td><td>30</td><td>32</td><td>8</td>
</tr>
<tr>
<td class="text-left">Califia Farms</td>
<td>20</td><td>30</td><td>32</td><td>8</td>
</tr>

<tr><th colspan="6" id="tcl376">Top five companies</th></tr>
<!-- Continue with similar rows as needed -->
</tbody>
</table>

Proposed resolution

Workaround: Disable the insertTable widget (looks like a square with many squares inside.

Remaining tasks

Create some sort of ck5 caveats documentation.

User interface changes

Preserve sane behavior with tables.

API changes

Driven by upstream ck5 widget/plugin called insertTable

Data model changes

Ruined markup.