If you design user experiences for standards-based websites and applications (i.e. those built with XHTML, CSS, and JavaScript), there are several great reasons for adding XHTML prototyping to your UX tool kit. Perhaps you’ve found that traditional wireframes just aren’t sufficient and are looking for more powerful ways to explore and communicate design solutions. Perhaps your current practice is based on the traditional waterfall model (i.e. first creating wireframes, which are handed off to creative, who hand off comps to tech, and so forth), and you want to explore more contemporary methodologies, such as agile and iterative development. Regardless, a great way to embark on that journey is to start prototyping with XHTML.