Notice that we do have two div elements with their respective opening/closing tags. It just so happens that the blueDiv element is nested inside the outer element. This nesting is an important part of making the centering work, so we can’t have them be peers like in your suggestion. If we did have them be peers, then you will need closing tags as follows:
No, the <div id="outer"> is the opening div tag. It just happens to have the id attribute in it, but that doesn’t negate the role of it still being the opening tag