@krusynth @matthiasott I don’t think that is correct.
According to the MDN "flow content" is allowed as children of <li> and <article> is in the list of flow content.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/li
https://developer.mozilla.org/en-US/docs/Web/HTML/Content_categories#flow_content
The HTML 5 spec has an example with article elements for comments and #webmentions are something like comments, right?
https://www.w3.org/TR/2013/CR-html5-20130806/sections.html#the-article-element