<xsl:if> element contains a test attribute and a template. If the test evaluates to true, the template is processed. In this it is similar to an if statement in other languages. To achieve the functionality of an if-then-else statement, however, use the
<xsl:choose> element with one
<xsl:when> and one
<xsl:if test=EXPRESSION> TEMPLATE </xsl:if>
Contains an XPath expression that can be evaluated (using the rules defined for
boolean( )if necessary) to a Boolean value. If the value is true, the template is processed; if it is not, no action is taken.
Instruction, appears within a template.
XSL section 9.1.