nope. it is not about xbl directly. there is already a nice working xbl schema at
it is more for custom elements. see the following
<button label="aButton" />
<button label="anotherButton" />
<myspecialbutton label="Button with special functionality created in XBL" />
all native xul-types, like button, box, etc. are written in xbl as well. thus you can also add your own xul elements, like the custom element in the example above. with the current implementation of your xul schema, you would get a warning when you insert such
an element into your xul and it would not be in the intellisense.
that's what my change is for. :)
Is it clear now?