XML actually does not do anything to the style sheet. XML is information wrapped in tags, so while it stores, structures and can be used to transport information, it doesn’t actually display the information it carries. Software outside of XML is needed to display the information it contains. This is hard concept to understand, but is what makes XML so powerful.
XML structured content can be displayed across different devices and platforms in a variety of styles and ways. The XML content sent to InDesign can look one way, but thanks to style sheets the very same content can look radically different displayed on a web page or in a mobile version. The content remains the same thanks to the XML structure. Rather than changing content to fit devices, XML can flow into style sheets for each specific device.
XML provides the structure; styling gets applied at the output channel so this: <headline> XML is Great! </headline>
Gets transformed into this: XML is Great!