By paring down the markup to it’s barest essentials, we make it easier for browsers, software, and all devices to read our content—at the same time making it easier for other designers and developers to modify and edit these components.
By removing as much complexity and providing as much common sense as possible, we can work better in groups and have confidence other professionals can pick up where we left off with the least amount of hiccups.
“Nonessential graphics” refers to graphics that don’t apply any meaning to the content, or that don’t provide any instruction or direction to the user… These graphics will just get in the way of someone trying to access this site via a text browser or screen reader.
A example later in this book was a rounded corner. There is no real reason graphic elements like that should be a part of the XHTML. As much as possible graphics of that nature should be delivered through CSS.