If it were Python-based but had nothing to do with Python, would you really care if it wasn't Python based?
Anyways, yes, a project exists. A pretty big one too. It's called XULRunner. The project is maintained by Mozilla and is used for the GUI of every Mozilla program.
It features an XML-based syntax (XUL):
<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<window id="main" title="My App" width="300" height="300" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<script type="application/javascript" src="chrome://myapp/content/main.js"/>
<caption label="Hello World"/>
<separator/>
<button label="More >>" oncommand="showMore();"/>
<separator/>
<description id="more-text" hidden="true">This is a simple XULRunner application. XUL is simple to use and quite powerful and can even be used on mobile devices.</description>
</window>
And JavaScript:
function showMore() {
document.getElementById("more-text").hidden = false;
}
You can even embed Python scripts, it seems, into your code: http://pyxpcomext.mozdev.org/no_wrap/tutorials/pyxulrunner/python_xulrunner_about.html