Should docutils whine about nesting simple body elements?

Nesting paragraphs in paragraphs is indeed a violation of the Docutils document model docutils.dtd.
Unfortunately, this is just one example of an "infinite" number of ways you can build invalid documents -- testing for all sorts of violations by custom code may seriously affect performance.

You may consider exporting a test document to XML and test for validity against the docutils.dtd.

Ticket moved from /p/docutils/bugs/466/

Unfortunately, this is just one example of an "infinite" number of ways you can build invalid documents -- testing for all sorts of violations by custom code may seriously affect performance.

However, a transform testing for conformance could be sensible. It may walk over the document tree and for each node check that the parent node is an instance of the well-defined, small set of allowed parent nodes.
Ideas and patches are welcome.

In Docutils 0.22 (since [r9732]), you can validate a Document Tree element (nodes.Element instance) and its children by calling its validate() method.
There is also a "transform" that checks a complete document after parsing and reports validation problems as WARNING system messages. It can be activated with the validate configuration setting.

The new "xml" parser validates the parsing result by default, so if you save the source

<paragraph classes="availability">
    <inline classes="xref std std-ref">Availability:</inline>
    Unix, not WASI.
    <paragraph>Content of the "availability" directive.</paragraph>
</paragraph>

to a file foo.xml, say, and run

docutils --parser=xml --writer=pseudoxml foo.xml

the result is

<document source="/tmp/foo.xml">
    <paragraph classes="availability">

        <inline classes="xref std std-ref">
            Availability:

        Unix, not WASI.
        <paragraph>
            Content of the "availability" directive.

    <section classes="system-messages">
        <title>
            Docutils System Messages
        <system_message level="2" line="4" source="/tmp/foo.xml" type="WARNING">
            <paragraph>
                Element <paragraph classes="availability"> invalid:
                  Child element <paragraph> not allowed at this position.

The original example would also complain about the unknown node <pending_xref> as this is an extension by Sphinx.