There are no known areas where LibXML/LibXSLT are not fully compliant with the relevant W3C specs (they're even authored by the W3C!). If you do know of areas where they fall down on this please do let me know and I'll forward the info on to Daniel.
Snippets of code should be wrapped in
<code> tags not<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor