Standard: http://www.w3.org/TR/xslt#element-preserve-space
preserve-space
Attributes: elements
Standard excerpt:
For source documents, the set of whitespace-preserving element names is specified by xsl:strip-space and xsl:preserve-space
top-level elements. These elements each have an elements attribute whose value is a whitespace-separated list of NameTests.
Initially, the set of whitespace-preserving element names contains all element names. If an element name matches a NameTest
in an xsl:strip-space element, then it is removed from the set of whitespace-preserving element names. If an element name
matches a NameTest in an xsl:preserve-space element, then it is added to the set of whitespace-preserving element names.