Elements Must Be Well-formed
First, each element must be closed. They are two ways to close an element as depicted below. They are equivalent.
|Close by an end tag:|
|Close without an end tag:|
Second, elements must be properly nested.
<window> <groupbox> Hello World! </groupbox> </window>
<window> <groupbox> Hello World! </window> </groupbox>
XML treats every tag as a node in a tree. A node without a parent node is a root component, and it is the root of a tree. In each zul file, only ONE tree is allowed.
For example, for being a whole zul file, the following is allowed, for it has only one root component.
And for being a whole zul file, the following is not allowed, for it has more than one root component.
You can solve the problem by simply adding a tag enclosing the whole zul file to serve as the parent node, so the zul file has one single tree again.
<window> <button /> <button /> </window>