The Web Design Layer Cake

Contemporary web design is commonly visualized as being made up of three separate “layers.”
The content of the document with its (X)HTML markup makes up the Structure Layer. It forms the
foundation upon which the other layers may be applied.
Once the structure of the document is in place, you can add style sheet information to control how the
content should appear. This is called the Presentation Layer.
Finally, the Behavior Layer includes the scripts that make the page an interactive experience.