There is a middle ground between using XML and imperative code for representing tax forms. Robert Sesek’s ustaxlib [0] uses JavaScript to encode the forms in a way that is reasonably statically analyzable. See the visualizer [1]. My approach uses XML to represent the forms with an embedded DSL to represent most expressions tersely. See for example Form 8960 in ustaxlib [2] and my TaxStuff program [3]. The main thing that the XML format from the article has going for it is that it is easy to write a parser for. But it is a bit verbose for my taste.
[0]: https://github.com/rsesek/ustaxlib
[1]: https://github.com/rsesek/ustaxviewer
[2]: https://github.com/rsesek/ustaxlib/blob/master/src/fed2019/F...
[3]: https://github.com/AustinWise/TaxStuff/blob/master/TaxStuff/...