Here's some suggested prose. The sections would need reordering to get the flow right.

The following browsers allow the use of [DOM mutation observers](http://dom.spec.whatwg.org/#mutation-observers) to access the HTML [microtask queue](http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#microtask-queue), and thus begin flushing ASAP's task queue immediately at the end of the current event loop turn, before any rendering or IO:

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The following browsers do not have mutation observers available, but do have [message channels](http://www.whatwg.org/specs/web-apps/current-work/multipage/web-messaging.html#message-channels), which give direct access to the HTML [task queue](http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#concept-task). This allows ASAP to begin flushing its own task queue immediately after any queued rendering or IO is performed.

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Note that in web workers, mutation observers are not available, and so even the browsers from the first list will fall back to the HTML task queue instead of microtask queue.

Finally, the following browsers support neither mutation observers nor message channels, necessitating a fallback to `setTimeout`. This generally delays the flushing of ASAP's task queue for four milliseconds.

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