XML in the Browser

XML + CSS This simple approach combines an XML document with a set of rules (the CSS style sheet) for presenting that document. It presents the document 'as is', without modifying the document structure.
XML + XSLT XSLT allows developers to transform XML documents into HTML or XML+CSS result documents, changing the structure of the document as necessary for a particular application.
HTML + "XML Data Islands" Microsoft's 'Data Islands' aren't kosher XML (you can't have elements named 'XML', for instance), but they do give developers working with Internet Explorer 5.x a means of including XML content inside of a document or referencing it externally.
Dynamic XML It hasn't really come to pass, thanks largely to a lack of standards for scripting XML in browsers, but it's thoroughly implemented in Mozilla, especially XUL, and somewhat implemented in Internet Explorer 5.x.
XHTML A recasting of HTML in XML syntax, XHTML makes it possible for HTML and XML developers to use each other's tools and vocabularies. XHTML 1.0 is a simple clean-up, while Modularization introduces tools for adding to the XHTML vocabulary and reusing portions of that vocabulary in other XML vocabularies.