var el = document.getElementById('element');

// Set will-change when the element is hovered
el.addEventListener('mouseenter', hintBrowser);
el.addEventListener('animationEnd', removeHint);

function hintBrowser() {
  // The optimizable properties that are going to change
  // in the animation's keyframes block
  this.style.willChange = 'transform, opacity';
}

function removeHint() {
  this.style.willChange = 'auto';
}