I'm not sure it would be xml, possibly xhtml which is a stricter version of html that conforms to xml standards.
The only way you really use xml is with an xml stylesheet (xsl). Most recent browsers will read xml file with xsl and display them as a web page. This is fairly common with rss xml feeds so they can be rad without an RSS reader.
Another use of xml is in web services which can be used to expose business data and services across the internet using an open standard. I have used this in cms systems where I want the data to be available to other sites and services.
maybe if you could point to where you have read this I could understand the context of this.