We present an efficient algorithm for testing approximate DTD validity modulo the strong tree edit distance. Our algorithm inspects XML documents in a probabilistic manner. It detects with high probability the nonvalidity of XML documents with a large fraction of errors, measured in terms of the strong tree edit distance from the DTD. The run time depends polynomially on the depth of the XML document tree but not on its size, so that it is sublinear in most cases. Therefore, our algorithm can be used to speed up exact DTD validators that run in linear time. We also prove a negative result showing that the run time of any approximate DTD validity tester must depend on the depth of the input tree.

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