What aesthetic effect do diagonal lines normally create in a building design?

They help spark an element of flow, intrigue, and energy. =)

i don’t know what it is

Well, this code would box the whole body of an html document. by using the relative value for position, you are telling it to position relative to its normal position, which the top left corner is at (0,0). by using left: 50px, you are telling the box to set the left margin  edge to 50px and by using top: 50px, you are telling it to set the top margin edge to 50px. by doing this shifting, the top left corner of the box, which is at (0,0) would be positioned at (50,50)