Documents served using an XML content type always get the Standards mode

And what about:

Despite the name “document type declaration” and despite what the HTML specification says about “version information”, the document type declaration is not an appropriate means for classifying SGML or XML documents as documents of a particular type even though it seems it was supposed to be (hence the name).

Being a perfectionist, I really like these kind of pedantic material.

I invite you to read read the whole article

Oh, and while you are here, get this one too:

HTML is a data format, not a programming language. You are allowed to use any programming language to write a loop that writes that text 100 times. You are also allowed to try to do that in HTML.

That is from here