According to the DITA 1.3 specification, the <lines> element represents
“text where line breaks are significant but white space is not”. 
However, XMLmind XML Editor (XXE) renders white space as significant.
And when ditac (dita converter) converts <lines> to HTML, the result is
that white space, again, is treated as significant.
It could very well be that the interpretation of ditac and XXE makes
more sense for the use cases that <lines> are meant for, but I would
nevertheless bring this to your attention and ask for a clarification.
Leif Halvard Silli
XMLmind XML Editor Support List