Updated Versioning compatibility strategies document

Hi all,

Based upon Henri's excellent feedback and many TAG member's previous
requests, I added some CSS material into the compatible strategies
versioning finding document.  This is a "compatible" change with the
previous version so if you reviewed the 20080912 version, your comments are
not invalidated.

http://www.w3.org/2001/tag/doc/versioning-compatibility-strategies-20080917.html
http://www.w3.org/2001/tag/doc/versioning-compatibility-strategies

Cheers,
Dave