* @param Object speed A string representing one of the three predefined speeds ("slow", "normal", or "fast") or the number of milliseconds to run the animation (e.g. 1000).
* @param Number opacity The opacity to fade to (a number from 0 to 1).
* @param Function callback A function to be executed whenever the animation completes.
* @param Object speed A string representing one of the three predefined speeds ("slow", "normal", or "fast") or the number of milliseconds to run the animation (e.g. 1000).
* @param Number opacity The opacity to fade to (a number from 0 to 1).
* @param Function callback A function to be executed whenever the animation completes.