the doctype does not get defined by the browser.
the doctype is defined by the DTD that it is pointing to, for HTML 4.0.1, the dtd path that is used is:
meaning that IE will look to the w3.org site to determine how to display the page.
It is valid to use the HTML 5 doctype for backward compatibility, at least for now, but there is no of knowing if that will be valid in 10 years.
Programers in the '70's thought it was valid to only use 2 digit years, look how that ended up.....
I dont think relaying on the HTML 5 doctype for backwards compatibility is a very good idea.
Yes, setting the User Agent using the meta tag is a good practice too, but not necessary for
the Spry Menu to work.