You only need node.js if you're going to be modifying the OpenSeadragon source. If you just want to use OpenSeadragon, grab a built version from http://openseadragon.github.io/#download. Then, assuming your project directory looks like so:
project/
dzi/
foo.dzi
foo_files/
...
index.html
openseadragon/
images/
...
openseadragon.js
openseadragon.min.js
Your index.html could look like so:
<html>
<head>
<style type="text/css">
#foo {
width: 400px;
height: 300px;
}
</style>
</head>
<body>
<div id="foo"></div>
<script src="openseadragon/openseadragon.min.js"></script>
<script>
var viewer = OpenSeadragon({
id: 'foo',
prefixUrl: 'openseadragon/images/',
tileSources: 'dzi/foo.dzi'
});
</script>
</body>
</html>
I guess I should add something like this as a "getting started" on the web site!