It's a rule in CSS to provide priority of interpretation to blocks of style. For example, if you have a DIV element with an ID "example" and a class name "example", and in the stylesheet you have: #example{ background-color: red; } .example{ background-color: green; } The DIV's background color would be red, violating the natural reading flow of the stylesheet (top down), because IDs have higher specificity than classes, which could not override the previous style selected by an ID.