You can of course buy and install a fully-fledged commercial XML editor with XSLT support, and run this application within it. However, this is beyond the reach of many users, so to do this unaided you just need to install three pieces of software: Java, Saxon, and the DocBook 4.2 DTD (URIs are correct at the time of writing). None of these has a visual interface: they are run from the command-line in the same way as is possible with \LaTeX.