Questionable handling of implied end tags

[Bob Kline]

According to https://html.spec.whatwg.org/multipage/grouping-content.html#the-dt-element

"A dt element's end tag can be omitted if the dt element is immediately followed by another dt element or a dd element."

Based on that language, I would have expected this

from lxml import html
html.tostring(html.fromstring("<dl><dt>one<dt>two</dl>"))

to have resulted in

b'<dl><dt>one</dt><dt>two</dt></dl>'

but instead I get this:

b'<dl><dt>one<dt>two</dt></dt></dl>'

... so the path of the second dt becomes dl/dt/dt instead of dl/dt as expected.

Note that, by contrast, the handling of li elements appears to match what the spec says:

"An li element's end tag can be omitted if the li element is immediately followed by another li element or if there is no more content in the parent element."

html.tostring(html.fromstring("<ol><li>one<li>two</ol>"))
b'<ol><li>one</li><li>two</li></ol>'

I tried pre-flighting this on the mailing list, but I've been getting strange mail delivery failures for my most recent messages:

*** MAIL DELIVERY FAILURE REPORT ***

The original message was received at Sun, 08 Mar 2020 09:12:09 -0700 (PDT)
from host by mail-wr1-x432.google.com with SMTP id n7so8011429wrt.11 for .
          <email address hidden>.

Subject: Questionable handling of implied end tags
From: <email address hidden>

Mail delivery to the following recipient has finally failed:

<email address hidden>
   Last reason: 550 5.1.0
   Explanation: host mxa.eu.mailgun.org [18.195.181.121] said: Recipient rejected:
                <email address hidden>

   Transcript of session:
   ... while talking to mxa.eu.mailgun.org [18.195.181.121]:
   >>> RCPT TO:<email address hidden>
   <<< 550 5.1.0 Recipient rejected: <email address hidden>

Seems odd that the recipient address doesn't match my own. At any rate, I'm going straight to the bug tracker. Here's the requested environment report:

Python : sys.version_info(major=3, minor=7, micro=6, releaselevel='final', serial=0)
lxml.etree : (4, 4, 2, 0)
libxml used : (2, 9, 10)
libxml compiled : (2, 9, 10)
libxslt used : (1, 1, 34)
libxslt compiled : (1, 1, 34)

That's on MacOS. Also reproducible on Linux and Windows, Python 3.8.0, lxml 4.5.0.