This is a good sniff to add, however it's too aggressive.
A violation should only trigger if:

• It's a list or table
• It's the only child element
• No text

For example the following is allowed.

 <para>
   The following constants are defined, and represent possible
   <function>exif_imagetype</function> return values:
   <table>
     <title>Imagetype Constants</title>
     ...
   </table>
 </para>

When building new sniffs it's important to measure performance. To get a more detailed measurement, you can use the --perf parameter.

Some things that we may consider here depending on what gives the least overhead.

• Use XPath (//path/table)
• or: get all table nodes first, and then check if it's parent is a <para>.

XPath would be the cleanest IMHO as it avoids a nested loop.

As commented, it is probably way faster to XPath search all "child" elements first, without //*[local-name()='para'], and then directly inspecting $node->parentNode->nodeName for "para"s. Simple searches and neighbour navigation is incredibly fast on libxml.

I ran the benchmark on the full doc-en reference directory (10 837 files) with both approaches:

• para → children (current): 0.008s
• elements → parentNode (your suggestion): 0.020s

The current XPath turns out to be ~2.5x faster, likely because there are far fewer nodes with direct block children to inspect than total occurrences of the disallowed elements across the whole tree.

Interessing. And go with the faster alternative.