not( expression )

expression

The expression is evaluated exactly as if it were passed as an argument to the boolean() function.

True for an expression that evaluates to false; false for an expression that evaluates to true.

• This function should behave similarly to the boolean() function except that it returns the opposite value.
• You can test if an element doesn't have some attribute.

  <xsl:for-each match="//a[not(@name and @name = 'badname')]">
    <!-- iterates over any <a> element in the document, that
          either has no 'name' attribute at all, or it has one,
          but its value is not "badname". -->
  </xsl:template>
  

XPath 1.0 4.3

Supported.