You’re right, I just took another look at the specs and corrected the template and added the “mediaType” attribute and set it to HTML. Now microblog.pub shows the summary correctly (even with rendered HTML), Pleroma shows the content correctly (without markup), but Mastodon shows the raw HTML code of the summary. 🤔
I think I finally got ActivityPub support for this blog working. On Mastodon, you can search for @email@example.com and @firstname.lastname@example.org to follow the English and German blog. You should also be able to search for the URL of any post and reply to it. But remember that it’s only possible for the reply to appear in the “Interactions” if your profile isn’t locked (so your reply is visible to the public). I will write about my implementation later, when I’m sure everything is working correctly.