This is a good beginner-friendly explanation of why HTML is more than just the markup we see on a page.

I especially liked the part about semantic HTML and how browsers parse HTML into a structured representation. When working with document rendering and browser-based content, I’ve found that the underlying structure becomes really important once you move beyond simple pages and start adding interaction.

The HTML/CSS/JavaScript separation is also a useful way to explain the web stack to beginners. A solid understanding of HTML makes the behavior of more complex web applications much easier to reason about.