If web pages were made out of wood, the grain would be running up and down. Vertical is the natural flow of web layout. When page elements reach the right edge of the browser window and go over, the flow defaults to “wrapping” that element down onto the next line. The more content on the page, the taller it gets, not the wider. Why is this? Because it just makes sense. Our eyes are used to reading fairly short lines of text, so if we were to see a paragraph of text in one long straight line, it would be painful to read. Paragraphs need to have line breaks in them to be readable (hence the term “blocks” of text). We sure as heck don’t want to hard-code line breaks into the markup ourselves. Obviously, we don’t have to, the browser does this wrapping for us. Thus the vertical expansion.