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.

Description

Code

Close by an end tag:

<window></window>

Close without an end tag:

<window/>

Second, elements must be properly nested.

Result

Code

Correct:

<window> <groupbox> Hello World! </groupbox></window>

Wrong:

<window> <groupbox> Hello World! </window></groupbox>