A valid XML document is one that conforms to the rules of its schema. oXygen makes it easy to verify that your document is valid and to fix problems with an invalid document.
In the upper right corner of the main window in Oxygen, a box indicates the validity of your XML document. A red box means that it is not valid, and a green box indicates that your document is valid.
In this example, the document is not valid because it has an ampersand ("Chapman & Hall"), and XML documents cannot include these.
The box in the upper-right hand corner of the editing screen will be red, and a mark in the vertical scrollbar will show you what part of the document makes it invalid.
Images courtesy of University of Michigan Library.
When we correct the error (by changing the ampersand to the Unicode "&"), the document becomes valid.