You can use layers in your Illustrator artwork to generate CSS layers in the resulting HTML file. A CSS layer is an element that has an absolute position and can overlap with other elements in a web page. Exporting CSS layers is useful when you plan to create dynamic effects in your web page.

The Layers palette in the Save For Web & Devices dialog box gives you control over which top-level layers in your artwork are exported as CSS layers, and whether exported layers are visible or hidden.