Certain property value pairs rule out other property value pairs, causing them to be ignored by the browser. For example, when an element has display: inline, any further declarations about width, height and margin-top properties will be ignored. Sometimes this is confusing: maybe you forgot that your margin-top will have no effect because the element has display: inline, so you spend a while struggling to figure out what you've done wrong. This rule protects against that confusion by ensuring that within a single rule you don't use property values that are ruled out by other property values in that same rule.