Mozilla developed the browser with the belief that they should be more customizable, and Chromeless is their pre-alpha experimental answer. The browser utilizes iframes instead of XUL, and applications are executed from an HTML file. This HTML accessibility allows developers to use standard technologies to essentially create their own browser.
This project is in the very early stages and changes are expected to happen quickly and often. Developers wanting to jump on the bandwagon immediately with Chromeless should keep this in mind.