In most browsers and with Intel based hardware you can only achieve 64 callbacks per second (or one callback every 15.6ms) even when using setTimeout(f, 0). [1]
Maybe the task of reflowing the document just takes significantly less than 15ms, so that it always finishes before the callback is executed. But I don't know how browsers are programmed so this is just a guess.
Maybe the task of reflowing the document just takes significantly less than 15ms, so that it always finishes before the callback is executed. But I don't know how browsers are programmed so this is just a guess.
[1] http://software.intel.com/en-us/articles/cpu-power-utilizati... (the " Interrupt Rate Granularity" section is the interesting part)