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)