Writing To Head Using Js And Dom

i've been reading up on access to the dom via js (i'm new to this dom
stuff)

i understand tutorials and info such as
http://developer.mozilla.org/en/doc...t.createElement

my problem: i'd like js to create the code for a stylesheet in the head
of the docuement!

i'm not sure how do do this (or if it can be done)

can i create the following...

<link rel="stylesheet" type="text/css"
href="mystyle.css" />

OR

<style type="text/css">
hr {color: sienna}
p {margin-left: 20px}
body {background-image: url("images/back40.gif")}
</style>

using the dom or anything else in js alone?...or am i fighting a losing
battle.

(what i really want at the end of the day is to detect the browser and
use a css file according to that file, either with js or php)...