Mozilla and its successors, Firefox and Thunderbird are actually built on XUL[1], which is an XML-based UI description language which can be mixed with HTML and SVG, styled with CSS and programmed just like HTML with JavaScript. Then there's the xulrunner, which lets you write standalone apps on this tech. It's never taken off much, but I don't know if that's down to flaky implementation or just lack of interest. (if you don't like XUL, just use a minimal amount of it to embed your HTML app)

[1] I'm not sure what the current status is, but I think they want to phase it out?