The reason why the middle body is written at the back is that the previous left right has the float attribute set and is out of the document flow. At this time, the following body will automatically move up to the original document flow position of left.margin-left=left's width , margin-right=right's width. So in fact, the width of the main body is the entire width of the screen. When the document flow placed in the main body is placed in the middle, it will cover the entire width, so the right will fall.