Image Update Algorithm

Hi,
In http://www.w3.org/TR/html5/embedded-content-0.html#update-the-image-data talks about update the image data algorithm. I have several questions regarding it
- How is this algorithm invoked? Does UI task source enqueue a task to invoke the algorithm?- In step 8 it says that asynchronously await a stable state. What is a stable state in this case? Why can not run the algorithm till step 13 and say run the remaining steps asynchronously.
ThanksRegards,Essam