I suppose you want
local storage. This is a really good article:
http://diveintohtml5.info/storage.html[
^].
Of course, you can store your data in xml also, but I doubt, that this could be a good idea. JSON representation looks more adequate - aside from some really special cases.
But of course, you can manipulate xml on client side with javascript. See some examples here:
XML and JavaScript[
^] or
JQuery[
^].