xsl:if element is used for conditional processing. It takes a mandatory
attribute, whose value is a boolean expression. The contents of the xsl:if element are expanded only
of the expression is true.
The full syntax of boolean expressions is outlined in XPath Expression Syntax.
Example:<xsl:if test="@preface"> <a href="preface.html">Preface</a> </xsl:if>
This includes a hyperlink in the output only if the current element has a