Congratulations to our monthly contest winner for November, David Noble of Orangeville, Ontario. So you might expect that at this stage of the XML life cycle most processors would conform quite well to the specification. Surprisingly, that is not the case. The class median on this open book test was about eighty percent, which suggests that many implementors just haven't applied any real effort to getting conformance right. One could expect that of early stage implementations, but those are not the ones exhibiting such problems most strongly.
Many processors reject legal documents.
This can be extremely troublesome in terms of interoperability. Several have problems handling UTF-16 encodings. Handling of XML characters that require use of Unicode surrogate pairs is as yet inconsistent. This is expected to change in the next few years, however. While that latter is probably just a common oversight, the former might be an attempt at false efficiency.