When the first edition of this book was written, many people were writing XML using simple text editors, such as Notepad for Windows, and TextEdit for the Mac. Then, in order to validate or transform their XML, they were using command-line or Web-based applications, neither of which were necessarily easy to use or easy to fit into a business workflow.
Luckily, things have changed. For starters, the current Web browsers (including Mozilla Firefox, Internet Explorer, Opera, and Safari), have support for XML and XSLT. You also have the option to choose from dozens of XML editors that you can download from the Internet and use right now. Many of these editors are free to use, and some of the free ones are actually very good.