Reported in #webkit, tested on multiple OS X 10.9 computers. It's not spinning CPU, as nothing shows up in the profiler.
For reference, "WebKit r169891 was built on 12 June 2014 and is a 59.4 MB download."
Also, r169763 works fine.
Fixed in r169900.
*** Bug 133816 has been marked as a duplicate of this bug. ***