Even though an emoji is nothing more than an image cut out, once you start adding features to it like an animation it will depend on it being recognized by the browser, as an animated svg or png, and inserted in the code as such.
Then there is the problem of browsers recognizing the emoji, as an emoji. which is not a given. Browsers will only display a custom emoji correctly, to its own support of how to do so.
Anything new, (and animated png is new in browser support terms) takes time to gain full x-browser support. This is the same for animated png and svg, all modern browsers say they support it, but they do not support it the same, a simple svg is fine, add animation and it then depends on how you add the animation. Add color and it depends how you add color, if its an animated png, not all browsers support them, (displays as standard png, no animation).
You will have to do the same as everything else concerning browsers, wait untill there is an established standard, then wait a further few years for it to become a recommendation, then if you are lucky browsers will start to offer better support. Until then, it is a create it and see if it works thing.
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