There is no XUL in there.

Last time I checked, the vbox/hbox is XUL. https://developer.mozilla.org/en/docs/XUL/vbox

Sure, but that's only in index.xul, which is a very small part of the project. Everything else is plain old js+html (xhtml necessary for localization). Apologies, I didn't notice that when I said "there's no XUL in there".

As a meta-discussion, <vbox> is just a fancy way of saying:

div { display: -moz-box; -moz-box-orient: vertical; }

...which is the old flexbox implementation. But I digress :)

edit: link to the MDN article about `display: box`: https://developer.mozilla.org/en-US/docs/Web/CSS/box-orient

We could probably rewrite the entire Firefox UI in HTML nowadays, but that'd mostly be busywork (and would break a bunch of extensions). I would bet that Servo's UI will just be HTML.

Servo is just the browser engine, like Gecko. The UI/browser-chrome will be a separate project (named Crow, but not started yet afaik). Also relevant is the fact that Servo wants to be embeddable, like Webkit.