You should be able to edit xml files as well, make sure the tags are properly opened and closed, if it is a invalid xml then it won't let you save the file.

On further investigation, the AA Workspace doesn't handle comments as expected. For instance:

This is "Valid" in the AA Workspace:

<!--

This is a comment,

possibly multiline.

-->

<gomer></gomer>

Whereas this is not valid:

<!-->

This is a comment,

possibly multiline.

<-->

<gomer></gomer>

While most XML formatters (and indeed, the formatter used by the view in the AA workspace) identify the second example above as valid, AA does not.