XPath is the preferred way of traversing a DOM in Fonto. Using XPath, you can easily look up a node, search for an attribute or compare the textual content of nodes.
XPath queries can be tested using the XPath test app, which can be found at https://xpath.playground.fontoxml.com.
See the XPath 3.1 spec for more information.
The implementation used by Fonto is open source, it can be found at the github page.