Possible bug report with HTML and MD about commonmark

[colinodell]

This is expected behavior according to the CommonMark specification. It's easier to explain what's happening here if we shorten your example Markdown input to this:

<ul style="position:absolute; top:-999px; left:-9999px;">
<li><a href="#toc1">Retourner aux généralités sur les extensions</a></li>
</ul>
<a id="listext"></a>
## Extensions de NVDA
### Amélioration du confort d'utilisation de NVDA
####[Add-on Updater](https://www.nvda-fr.org/article.php?id=48)
Cette extension permet de vérifier, de télécharger et d'appliquer les mises à jour des extensions.

The headings are not being converted because you don't have a blank line in between the HTML and the headings. According to the CommonMark spec, this type of HTML block will continue indefinitely until a blank line is found.

You can avoid this by adding a blank line like this:

<ul style="position:absolute; top:-999px; left:-9999px;">
<li><a href="#toc1">Retourner aux généralités sur les extensions</a></li>
</ul>
<a id="listext"></a>

## Extensions de NVDA
### Amélioration du confort d'utilisation de NVDA
####[Add-on Updater](https://www.nvda-fr.org/article.php?id=48)
Cette extension permet de vérifier, de télécharger et d'appliquer les mises à jour des extensions.

You may find it helpful to use the official CommonMark dingus or Babelmark3 to explore how certain Markdown is converted and whether this library deviates from the spec.