Forward compatibility html to x html with mod rewrite?

With the Apache URL Rewriting Guide is is relative easy to make a website backward compatible. See http://httpd.apache.org/docs/2.0/misc/rewriteguide.html .

But is it also possible to make it "forward compatible"? That is serve all html-pages as xhtml if the browser accepts xhtml. So all pages remain *.html for IE and present links, but are altered on the fly to xhtml with the correct mime type application/xhtml+xml. Of course, all pages are correct xhtml files.

The question is how? My try is below, but doesn't not work.

AddType application/xhtml+xml .xhtml

<Directory "/var/www/alforto.nl">
Options +FollowSymLinks
RewriteEngine on
# Our conditions are that the browser must accept XHTML
RewriteCond %{HTTP_ACCEPT} application/xhtml\+xml
# If it accepts xml than rewrite html to xhtml
RewriteRule *.html$ *.xhtml [L]
</Directory>


Arjen