The most difficult step of an XML-centric design process is the specification of the XSL specifications. However, if the DTD is well defined and designed, the design of an XSL specification is often straightforward and logical.
Note that writing an XSL specification is kind of like programming. As a result, not everyone who can code in HTML can write XSL without further training. Nonetheless, the combination of DTD-XML-XSL is a bit more powerful and expressive than HTML. In the long run, it is worth the time and effort to learn XML and its related components (DTD and XSL).
Testing XSL is, surprisingly, not very difficult. This is because most modern browsers have built-in XSLT engines. Firefox, Internet Explorer and Opera all have built-in XSLT engines, which means they can take an XML document and an XSL specification and render the HTML presentation to be displayed.
Combined with accessibility assessment tools like Firevox, Fangs and Accessibility Extension, writing XSLs for normal (media-rich) delivery and accessible delivery from the same DTD is not necessarily a daunting task.