In the previous chapters of our HTML course, we learned extensively about HTML elements and, in most cases, nested some inside others. All webpage elements are related in one way or another, with clearly defined relationships.
When we learned about header and footer tags in our HTML Body Fundamentals chapter, we created both elements and placed others inside them. For example, the header tag had <div>, <ul>, and <li> elements resting inside it, with the <li> tags sitting inside the <ul> tag.
Their presentation shows some relationship, and that's part of what forms the HTML document tree.