Ask YC: Ajax Basics Help

I am currently learning the basics of Ajax and going through tutorials. Right now, I'm learning how to build a simple RSS feed reader. All of the examples I have seen so far, however, use getElementByTagName() to go through the XML document by hard coding in each of the tag's names. This doesn't seem right to me, though; shouldn't I just be able to use the JavaScript DOM instead and write a script that would work regardless of XML input?

Here is the example I am working with: http://www.w3schools.com/Ajax/tryit.asp?filename=tryajax_httprequest_js4

Any help would be appreciated.