​Yes.

In testing XML Schema and example XML when building tools [1] for S3Model [2].

In this white box testing, for me it is a relatively manual process even though I do use some Python unit testing as well Selenium for automated browser testing.

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How? In short you break things to be sure your test cases and your schemas catch  the errors. This paper may be of some interest https://ieeexplore.ieee.org/abstract/document/1510064/

[1] https://datainsights.tech/Kunteksto/index.html

[2] https://datainsights.tech/S3Model/