A layer is a container in a web page that can hold page elements, such as text and graphics. You can overlap, nest, and show or hide layers on a page. Layers can be used to create web page layouts and, when combined with Behaviors, to create animations and fly-out menus. Layers can be viewed in browsers that support HTML 3.2 and later (for instance, Microsoft Internet Explorer 4.0 or later).
A layer is defined in Microsoft Expression Web as a set of <div> tags (representing the div HTML element) whose position property is set to either relative or absolute. The default layer that is generated by Expression Web contains an inline CSS that positions the layer absolutely, and sets the dimensions of the layer, its z-index value, and assigns the layer a unique ID.
After adding a layer to a web page, you can modify these properties and add new properties to the layer by using the layer and CSS features in Expression Web. You can create layers using the Layers task pane, the Insert menu, or the Toolbox task pane.