SideQuest HOWTO (Updated)
A handy HOWTO for setting up and using SideQuest.
Reddit thread where the Textual Markdown widget was announced.
Also includes the rather hilarious comment "I wish there were more full blown app examples out there", back when Textual was pretty damn new and barely out of the initial "CSS branch" release.
Visual design rules you can safely follow every time.
You do not have to follow these rules every time. If you have a good reason to break any of them, do. But they are safe to follow every time.