Ah, this appears to do the trick:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_ACCEPT} application/xhtml\+xml
RewriteRule \.html$ - [T=application/xhtml+xml]
</IfModule>

>>6
the firefox displays an error page that says "not well formed"... the w3c validator shows <nobr> as being the only problem, and removing it makes the error go away in firefox....

>>7
that only works if you have mod_rewrite, and 100webspace apparently doesn't...