Symphony.

Even using DocType Xhtml 1.1 inside xsl:output.

<xsl:output method="xml"
 **doctype-system="-//W3C//DTD XHTML 1.1//EN"**
 doctype-public="http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"
 omit-xml-declaration="no"
 media-type="application/xhtml+xml"
 encoding="UTF-8"
 indent="yes" />

The output of curl -I site.domain.com is:

HTTP/1.1 200 OK
Date: Mon, 30 Dec 2013 21:14:01 GMT
Server: Apache
Cache-Control: no-cache, must-revalidate, max-age=0
Expires: Mon, 12 Dec 1982 06:14:00 GMT
Pragma: no-cache
Set-Cookie: PHPSESSID=knfa2gojbp5fjd96e2qer6mbf5; expires=Mon, 13-Jan-2014 21:14:01 GMT; path=/; domain=site.domain.com
Last-Modified: Mon, 30 Dec 2013 21:14:01 GMT
Vary: Accept-Encoding
**Content-Type: text/html**; charset=utf-8

Could someone help me explain this please?

See here

Thanks @michael-e, that provides a solution.

Do you know why symphony serves text/html by default, instead of its natural layout language (xhtml)?

text/html ist the correct Content-Type for HTML (4), XHTML (1.0) and HTML 5 — so it does cover XHTML ouput.

As far as I know, using XHTML 1.1 output has zero practical advantages over XHTML 1.0 strict. Why do you think it's better?

I started using xhtml 1.1 due to its modular design, but I'm actually transitioning to [X]HTML 5.

As to what you mention about xhtml 1.0, actually if the content is sent as text/html, no browser would really interpret this as xhtml. Instead it would be interpreted as html (even if it is xhtml 1.0). https://developer.mozilla.org/en-US/docs/XHTML

I have understood to be a compatibility issue with lnternet Explorer 8 and above.

But, really thanks for the extensions refered, It was really helpful. http://www.getsymphony.com/download/extensions/view/20040/ http://www.getsymphony.com/download/extensions/view/90662/

If you want to output HTML 5, be sure to check the HTML5 Doctype extension. It's the most flexible way to do it, IMHO.