xsltproc/LibXSLT - non-compliance?

    As for Gestalt versus xsltproc [...] Xsltproc will run faster,
    but is non-compliant to the XSLT 1.0 recommendation.
    -- http://www.stylusstudio.com/xsllist/200701/post40510.html

    [...] when I still used XSLT [1.0, M. L.], I would use xsltproc
    for my processor, due to its famed speed (I wasn't aware of its
    non-compliance at the time) [...]
    -- http://colina.demon.co.uk/?q=node/20

What exactly does xsltproc's (that is: LibXSLT's) non-compliance (with
the spec, I guess) consist in? And is there a standard test suite of
XSLT programs to determine a processor's compliance?

The other day I noticed that for a particular case, xsltproc produces a
different result than its fellow processors. Has this difference been
determined in more general terms?