Playgrounds are offered to let developers play and experiment with isolated parts of FontoXML. Currently, there are two different playgrounds available.
The FDS Playground contains all components exposed by FDS. It can be found on https://fds.playground.fontoxml.com/.
XPath Test App
The XPath Test App is a tool to test XPath queries on a stubbed DOM. It is available on https://xpath.playground.fontoxml.com/. Please do note that this playground always uses the latest version of fontoxpath, which might be more recent than the one included in your application.