What Was the HTML Concept That Took You the Longest to Understand?

When I first started learning HTML, I thought it would be the easiest part of web development. The basic tags seemed straightforward, and creating a simple page didn't take long. But as I kept learning, I realized that HTML is more than just putting elements on a page. Understanding semantic HTML , proper document structure, forms, accessibility, and knowing which element to use in different situations takes practice. For example, it can be tempting to use for almost everything, but learning when to use elements like <section>, <article> , <nav> , or <header> makes a big difference in writing meaningful markup. I'm curious about other developers' experiences: What HTML concept confused you the most when you were starting out, and what helped it finally click? I'd love to hear different perspectives, especially from people who have been working with HTML for a while.