Yes, I wasn't very clear in my question, sorry. When I created a page I was expecting to this code:
!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
That is what I am referring to.
Is that code even nessecary?
No, that code isn't necessary because Muse exports HTML5.
The doctype you've posted there is for XHTML Transitional.
Here's more information about the various doctypes: http://www.w3schools.com/tags/tag_doctype.asp
But remember, Muse takes care of the code for you, so you should never need to add a doctype to your pages.