Yes, .htm, .html can be used to serve XHTML 1.1, but it's best to use .xhtml instead. This way, one can add filters to sniff IE and send it "ContentType text/html" instead of "ContentType application/xhtml+xml". Otherwise, you would send "ContentType application/xhtml+xml" on non XHTML documents to Firefox, Opera, et. al.

This also applies to Server Side Includes. Keep .shtm, and .shtml for HTML and non-XHMTL 1.1 documents. Use .sxhtml for XHTML 1.1 documents.

This is not complicated. All one has to do is to use .htaccess or CPanel to add a directive to the Apache configuration. The problem is some editors. Dreamweaver 8 will not highlight .sxhtml documents. No such problem in TopStyle or other editors that allow you to add file extensions easily.

I have been modifying the MMDocumentTypeDeclarations.xml and MMDocumentTypes.xml in "C:\Program Files\Macromedia\Dreamweaver 8\Configuration\DocumentTypes", but it still will not work.

PS: Dreamweaver also, by default, has a wrong template for XHTML 1.1. It misses the xml declaration and has the wrong ContentType of "text/html".

Example: index.sxhtml (It's on a laptop, and so, it might turn off).

Keen eyes will notice that it's not being sent as "application/xhtml+xml". I think that it might be a bug in the way Lighty handles SSI.

This is what I have done to configure LIghty:

mimetype.assign = (
	".xhtml" => "application/xhtml+xml", 
	".sxhtml" => "application/xhtml+xml")

$HTTP["useragent"] =~ "MSIE" {
	mimetype.assign = (
		".xhtml" => "text/html",
		".sxhtml" => "text/html")
}

ssi.extension = (".shtm", ".shtml", ".sxhtml")

For non server-parsed files, it WORKS.

Indeed, you should be using content negotiation and you should also be using URIs without file extensions : )

Another article to read: http://www.w3.org/QA/2006/02/content_negotiation.html

There are a few CSS issues, an DOM is totally different ( no shortcuts ). I'm not serving two different documents for different browsers. I'm serving the same document to different browsers, just different mime types.

But, file extensions are useful, and by no means, those are not clean URLs.

Content negotiation is exactly what I'm doing there.

This post is about .SXHTML, not specific servers.
That said, this works fine Apache.

# Prevent viewing of .htaccess files

	order allow,deny
	deny from all


# MIME types
AddType "text/html; charset=utf-8" .xhtml .sxhtml

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_ACCEPT} application\/xhtml\+xml
RewriteCond %{HTTP_ACCEPT} !application\/xhtml\+xml\s*;\s*q=0
RewriteCond %{THE_REQUEST} HTTP\/1\.1
RewriteRule \.xhtml$|\.sxhtml$ - "[T=application/xhtml+xml; charset=UTF-8]"

# SSI Handler
AddHandler server-parsed .sxhtml
Options Indexes FollowSymLinks Includes

# Index pages
DirectoryIndex index.php index.sxhtml index.shtml index.xhtml index.html index.htm

I'm not serving two different documents for different browsers. I'm serving the same document to different browsers, just different mime types.

If you're serving two different MIME types, you are serving two different documents ;)

Create an HTML document. Leave the quote off one attribute value.
Negotiate to send HTML 4.01 with text/html to one browser and then send XHTML 1.1 with application/xhtml+xml to another browser.

You see two different documents, do you not?