Free XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition by Michael Kay Page A

earlier, the main innovation is the ability to undeclare a namespace, using syntax of the form xmlns:prefix=“” . This is particularly intended for applications like SOAP messaging, where an XML payload document is wrapped in an XML envelope for transmission. Without namespace undeclarations, there is a tendency for namespaces used in the SOAP envelope to stick to the payload XML when this is removed from the envelope, which can cause problems—for example, it can invalidate a digital signature attached to the document.
    XSLT and CSS
    Why are there two stylesheet languages, XSL (that is, XSLT plus XSL Formatting Objects) as well as Cascading Style Sheets (CSS and CSS2)?
    It's only fair to say that in an ideal world there would be a single language in this role, and that the reason there are two is that no one was able to invent something that achieved the simplicity and economy of CSS for doing simple things, combined with the power of XSL for doing more complex things.
    CSS is mainly used for rendering HTML, but it can also be used for rendering XML directly, by defining the display characteristics of each XML element. However, it has serious limitations. It cannot reorder the elements in the source document, it cannot add text or images, it cannot decide which elements should be displayed and which omitted, neither can it calculate totals or averages or sequence numbers. In other words, it can only be used when the structure of the source document is already very close to the final display form.
    Having said this, CSS is simple to write, and it is very economical in machine resources. It doesn't reorder the document, and so it doesn't need to build a tree representation of the document in memory, and it can start displaying the document as soon as the first text is received over the network. Perhaps, most important of all, CSS is very simple for HTML authors to write, without any programming skills. In comparison, XSLT is far more powerful, but it also consumes a lot more memory and processor power, as well as training budget.
    It's often appropriate to use both tools together. Use XSLT to create a representation of the document that is close to its final form, in that it contains the right text in the right order, and then use CSS to add the finishing touches, by selecting font sizes, colors, and so on. Typically, you would do the XSLT processing on the server and the CSS processing on the client (in the browser); so, another advantage of this approach is that you reduce the amount of data sent down the line, which should improve response time for your users and postpone the next expensive bandwidth increase.
    XSLT and XML Schemas
    One of the biggest changes in XSLT 2.0, and one of the most controversial, is the integration of XSLT with the XML Schema language. XML Schema provides a replacement for DTDs as a way of specifying the structural constraints that apply to a class of documents; unlike DTDs, an XML Schema can regulate the content of the text as well as the nesting of the elements and attributes. Many of the industry vocabularies being used to define XML interchange standards are specified using XML Schema definitions. For example, several of the XML vocabularies for describing music, which I alluded to earlier in the chapter, have an XML Schema to define their rules, and this schema can be used to check the conformance of individual documents to the standard in question.
    When you write a stylesheet, you need to make assumptions about the structure of the input documents it is designed to process and the structure of the result documents it is designed to produce. With XSLT 1.0, these assumptions were implicit; there was no formal way of stating the assumptions in the stylesheet itself. As a result, if you try applying a stylesheet to the wrong kind of input document, the result will generally be garbage.
    The idea of linking XSLT and XML Schema was driven by two main considerations:
There should, in principle,