Layers were the core of a method of dynamic HTML programming specific to Netscape 4. Each layer was treated a separate document object in javascript. The content could be either included in the same file within the non-standard <LAYER> tag (or any other tag with the positioning set to "absolute" via CSS, loaded from a separate file with <LAYER SRC=url> or <DIV SRC=url>, or generated via javascript with the new Layer(), the content would then be filled with layer.document.write(). In modern browsers, this functionality is provided by using an absolutely-positioned DIV, or, for loading the content from an external file, an IFRAME.
References
- Netscape: Dynamic HTML in Netscape Communicator (On the Internet Archive)
- Eric Krock, "Transitioning from Proprietary DOMs and Markup to W3C Standards: Enhancing Pages That Use LAYER, document.layers[], and document.all to Support Standards"
