Michael,
Serna formats the file according to the whitespace stripping policy specified in the stylesheet (xsl
reserve-space, xsl:strip-space). If you want Serna to keep spaces everywhere as-is, you can use <xsl:strip-space="*">. For most document types, whitespaces betwen elements are insignificant in most cases.
Also, Serna does consistent formatting - so if you save document formatted by Serna to text-based CMS, another revision of the document will have little difference.
Of course you can insert newline character - either in whitespace-preserved area (in element which is listed in xsl
reserve-space, and for which XSL-FO whitespace treatment policy is 'preserve' too). Please look to the Interactive Example stylesheet, for the template of 'pre' element, as an example.