docs: function(offset, composite, easing) {
    ASSERT_ENABLED && assert(
        typeof offset === 'number' || offset === null,
        'Invalid offset value');
    ASSERT_ENABLED && assert(
        composite === 'add' || composite === 'replace' || composite === null,
        'Invalid composite value');
    this.offset = offset;
    this.composite = composite;
    this.easing = easing;
    this.cssValues = {};
  }