The generation of the pdf takes place "behind the scenes" on a special server which executes all latex commands. Due to dynamic pattern recognition the server knows when to re-run certain commands and can, as a result, assure that the generated pdf is created and displayed correctly (some latex editors require to run several commands more than once).
Daniel Kuster, founder of verbosus.com, says that it is necessary to "offer the users of a web application the same functionality as their stand-alone counterparts"
The service is offered at www.verbosus.com and can be used for free.
