Newline characters are most certainly allowed in well-formed XML.

How do you insert a line break?

To add spacing between lines or paragraphs of text in a cell, use a keyboard shortcut to add a new line. Click the location where you want to break the line. Press ALT+ENTER to insert the line break.

How do you break a line in HTML?

The <br> HTML element produces a line break in text (carriage-return).