Answer - There are two ways of centering block level elements: 1. By setting the properties margin-left and margin-right to auto and width to some explicit value: BODY {width: 30em; background: cyan;} P {width: 22em; margin-left: auto; margin-right: auto} In this case, the left and right margins will each be four ems wide, since they equally split up the eight ems left over from (30em - 22em). Note that it was not necessary to set an explicit width for the BODY element; it was done here to keep the math clean. Another example: TABLE {margin-left: auto; margin-right: auto; width: 400px;} In most legacy browsers, a table's width is by default determined by its content. In CSS-conformant browsers, the complete width of any element (including tables) defaults to the full width of its parent element's content area. As browser become more conformant, authors will need to be aware of the potential impact on their designs. If background and color should always be set together, why do they exist as separate properties? There are several reasons for this. First, style sheets become more legible -- both for humans and machines. The background property is already the most complex property in CSS1 and combining it with color would make it even more complex. Second, color inherits, but background doesn't and this would be a source of confusion.