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Preprocessor.processResource catches Exception errors properly, but ignores Error errors. When an Error gets raised internally, the mobileOK Checker crashes silently and enters an infinite loop: the thread keeps running forever. As it happens, the CSS validator may raise an Error. Solution: catch Error or the more generic Throwable class in Preprocess.processResource. (Plus apply the fix to other parts of the code that use multithreading)
"Throwable" errors now get caught properly in processResource. I could not find other parts of the code where a similar problem might arise.