How to play a animation with js after glam ready?

.play{
    animation-duration:.8s;
    animation-name:jump;
    animation-iteration-count:infinite;
    animation-timing-function:ease-in-out;
}
@keyframes jump {
    0%  { transform:translateY(0); }
    50% { transform:translateY(0.3); }
    100%{ transform:translateY(0); }
}

obj.addEventListener('viewover', function(e) {
    e.target.className = 'play';
});

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it do not work~
