The xsl:preserve-space
element is used at the top level of the stylesheet to define elements in the source
document for which white-space nodes are significant and should be retained.
The elements
attribute is mandatory, and defines a space-separated list of element names.
The value "*" may be used to mean "all elements"; in this case any elements where whitespace is not
to be preserved may be indicated by an xsl:strip-space element.