_elementKeyframeTransforms(element, rules, scopeSelector) {
    let keyframesRules = rules._keyframes;
    let keyframeTransforms = {};
    if (!nativeShadow && keyframesRules) {
      // For non-ShadowDOM, we transform all known keyframes rules in
      // advance for the current scope. This allows us to catch keyframes
      // rules that appear anywhere in the stylesheet:
      for (let i = 0, keyframesRule = keyframesRules[i];
           i < keyframesRules.length;
           keyframesRule = keyframesRules[++i]) {
        this._scopeKeyframes(keyframesRule, scopeSelector);
        keyframeTransforms[keyframesRule['keyframesName']] =
            this._keyframesRuleTransformer(keyframesRule);
      }
    }
    return keyframeTransforms;
  }
@param {Element} element
@param {StyleNode} rules
@param {string} scopeSelector
@return {Object}