This is the source of the short book Upgrade Your HTML.
(Note: The book is written using Markua, which is a superset of Markdown. Some of that code, like I> code for info boxes, is not rendered well by GitHub.)
Official description:
HTML is super-popular. Everyone can write HTML. Or can they? In the spirit that everything can be tweaked and optimized, Upgrade Your HTML is a first light book in a playful series to review and improve real-life examples of HTML.
Not shyly but always constructively does Jens Oliver Meiert, someone who has written a lot of HTML and who makes his own life difficult so that he can write even more HTML, go through ten samples to ponder and upgrade the respective markup.
If you’re a web developer, you know HTML. Check out and follow Upgrade Your HTML to nod (or shake your head) exploring old and new problems surrounding uses of the beloved HyperText Markup Language.
→ This is the book if you enjoy the intricacies of working with HTML.