"But since the installed base of server software generally determines the
content type of a document before opening the file, we suggest that authors use
a .html3 filename extension, or .ht3 on MS-DOS filesystems.
If the cost of deploying this convention at a site is too high, a server
administrator may assume that all documents with the .html extension may have
level 3 features, and label them as text/html; level=3, or convert them as
above. "
Of course, Paul Burchard might want ".html3d" for CyberView and VRML
experiments... :)
Rohit