There is a whitespace bug in the libxml2 version 2.9.4 used by testbot - some HTML tags will be collapsed together without any whitespace. I haven't tracked down exactly which version resolved the issue, but it may be "Initialize keepBlanks in HTML parser" which was a bugfix in 2.9.5
See e.g. https://bugs.php.net/bug.php?id=78129 for a reproduce case. echo filter_dom_serialize(filter_dom_load($markup)); can be called on the markup to reproduce.
Whatever is the easiest way to patch/upgrade libxml2 should resolve this, e.g. upgrading to debian bullseye?
Figure out how to resolve and create a patch / MR.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
probably it's outdated as 🐛 Upgrade filter system to HTML5 Fixed
I guess the lesson learned is that putting exact markup in test assertions is not a super great idea. This is the minor contortions I went thru to handle markup generated by different versions of drupal and libxml: https://git.drupalcode.org/project/mytube/-/blob/8.x-1.x/tests/src/Funct...