MutationObserver test page

Clicking start will schedule two self-restarting functions:

• runSetTimeout uses setTimeout(fn, 0) for queueing itself
• runMutation uses a MutationObserver for queueing itself

After starting the test, move your mouse so that it is hovering over the top line of output.

If the browser is behaving, you should see runSetTimeout calls interleaved with runMutation calls (either 2:1 or 1:1, depending).

On IE10, if our MutationObserver polyfill uses setImmediate, postMessage, or MessageQueue, the runMutation line will become erratic, and eventually not fire at all.