@Izzy I believe you would be able to post some interesting info to this question. The reason I'm saying is because bounty offerer (henceforth BO here) mentioned the site: https://www.cs.princeton.edu/research/areas as one of the sites not following the user-agent of the browser under all circumstances.
Now, your website https://android.izzysoft.de/articles.php also adjusts itself based on the screen resolution regardless of user-agent.