Why do W3C specifications have "implementation defined" parts?

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

In many of the W3C specifications there are parts that are "implementation defined."

To give one example, in the XSLT specification it says this about the <xsl:message> element [1]:

    The xsl:message instruction sends a message in an implementation-defined way. 
    The xsl:message instruction causes the creation of a new document, which is 
    typically serialized and output to an implementation-defined destination.


QUESTIONS:

1. The purpose of a specification is to define a standard. Doesn't "implementation defined" defeat that purpose since it means that each vendor will define their implementation in their own way, thereby obviating standardization?

2.Is this unique to the W3C? Do other standards organizations also have "implementation defined" sections in their specifications?


/Roger

[1] http://www.w3.org/TR/2007/REC-xslt20-20070123/#message