1. Create a table
2. Add a caption
3. Select caption and format it to be a heading (h1, h2..)
4. Note that the layout becomes distorted.
It is expected that the heading element will be wrapped in the caption element, which is valid html5 according to w3 validator
http://validator.w3.org/#validate_by_input
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<table style="width:1177px;"><caption><h2>Text</h2></caption><tbody><tr style="height:21px;"><td style="width:382.812px;">a</td>
<td style="width:382.812px;">a</td>
<td style="width:381.812px;">a</td>
</tr>
<tr style="height:21px;"><td style="width:382.812px;">a</td>
<td style="width:382.812px;">a</td>
<td style="width:381.812px;">a</td>
</tr>
</tbody></table>
<p> </p>
</body>
</html>
This behaviour has already been discussed with Yanko Dzhemerenov internally.