According to the stats of my Web site, there are still quite a few users who use old browsers like Netscape 3.0 and Explorer 3.0. If I start using XML on my Web site, will my site be compatible with these older browsers?
I would not currently recommend that you use XML on the client if you're anticipating targeting older browsers. Instead, you should think about using XML on the server. Then you can use XSLT to transform it into HTML for the client before sending it down. This will give you much more control over your output, and it will insure backward compatibility.