Keeping Whitespace In ResponseText, Etc.

I'm trying to access the source of an HTML page with as few alterations
from the actual source (as in, that seen from the View Source option)
as I can. The method document.documentElement.innerHTML returns the
HTML source, but adds HEAD and other elements if they are absent from
the source, and takes out whitespace (i.e., line feeds, carriage
returns and tabs) within tags and between tags. The follow function:

function xhr() {

xhr = new XMLHttpRequest()
xhr.open("GET","test-page.html",true);
xhr.onreadystatechange = function() {
if (xhr.readyState==4) {
alert(xhr.responseText);
}
}
xhr.send(null)
}

doesn't add or alter any tags that are absent in the source, and does
not take out line feeds within tags; it does, however, still take out
all non-line-feed whitespace within tags and all whitespace in general
between tags.

It seems that preserving whitespace is all that I need, but I haven't
found a way to do that through my searches. So is there any way to get
the unaltered HTML source of a page without innerHTML or applets, like
a better version of the XMLHttpRequest object's responseText method?