Chromeless is an experimental way of letting you build a browser UI using nothing but HTML but this could be the way of the future making web apps look exactly how you want them to look.
The overall architecture of Firefox is still something of a well kept secret in the wider computing world. It uses an application framework that renders the user interface as specified using XUL, an XML based markup language. You can use XUL and its associated code framework XPCOM to build your own applications which can be run using XULRunner.
The latest, admittedly early, beta supports the following features:
Track the page load progress of inner browsers
Receive notification about the security state of Web pages (whether a page is served over SSL)
Capture images from DOM fragments
Toggle to full screen mode
Log messages to the console
Save data to the user profile
Convert user input into valid URLs
An experimental API to support the Background loading of web pages
Support for Windows, OS X, and Linux
As well as providing a way to experiment with browser user interfaces it also paves the way to a browser that works exactly like a web page. After all XUL is similar to HTML and it works with a DOM just the same as the web page being displayed. Why not try substituting HTML for XUL?
This would means that the browser's UI was no different from any web page and would bring an element of "eating their own dog food" to the browser builders. More interestingly it would mean that the browser's UI could be customised as part of the process of displaying a web page. So if you visit a site offering a word processor app then the browser would adapt to provide a word-processor UI with none of the confusing non-application elements such as the back button or re-load.
If you would like to know more see the video that Mozilla Labs have prepared: